The Collaborative International Dictionary of English v.0.48:

Obligation \Ob"li*ga"tion\, n. [F. obligation. L. obligatio. See
   Oblige.]
   1. The act of obligating.
      [1913 Webster]

   2. That which obligates or constrains; the binding power of a
      promise, contract, oath, or vow, or of law; that which
      constitutes legal or moral duty.
      [1913 Webster]

            A tender conscience is a stronger obligation than a
            proson.                               --Fuller.
      [1913 Webster]

   3. Any act by which a person becomes bound to do something to
      or for another, or to forbear something; external duties
      imposed by law, promise, or contract, by the relations of
      society, or by courtesy, kindness, etc.
      [1913 Webster]

            Every man has obligations which belong to his
            station. Duties extend beyond obligation, and direct
            the affections, desires, and intentions, as well as
            the actions.                          --Whewell.
      [1913 Webster]

   4. The state of being obligated or bound; the state of being
      indebted for an act of favor or kindness; -- often used
      with under to indicate being in that state; as, to place
      others under obligations to one.
      [1913 Webster]

   5. (Law) A bond with a condition annexed, and a penalty for
      nonfulfillment. In a larger sense, it is an acknowledgment
      of a duty to pay a certain sum or do a certain things.
      [1913 Webster]

   Days of obligation. See under Day.

   under obligation, under an obligation. in a state of
      obligation[4].
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

obligation
    n 1: the social force that binds you to the courses of action
         demanded by that force; "we must instill a sense of duty in
         our children"; "every right implies a responsibility; every
         opportunity, an obligation; every possession, a duty"- John
         D.Rockefeller Jr [syn: duty, responsibility,
         obligation]
    2: the state of being obligated to do or pay something; "he is
       under an obligation to finish the job"
    3: a personal relation in which one is indebted for a service or
       favor [syn: obligation, indebtedness]
    4: a written promise to repay a debt [syn: debt instrument,
       obligation, certificate of indebtedness]
    5: a legal agreement specifying a payment or action and the
       penalty for failure to comply

Moby Thesaurus II by Grady Ward, 1.0:

183 Moby Thesaurus words for "obligation":
   accountability, accounts payable, accounts receivable,
   act of grace, act of kindness, affair, agreement, allegiance,
   amount due, appreciation, appreciativeness, aptitude, aptness,
   arrearage, assigned task, attempt, bad debts, benefaction, benefit,
   benevolence, benignity, bill, bills, blessing, bond, borrowing,
   boundary condition, bounden duty, burden, business, call,
   call of duty, catch, cause, chance, charge, charges, chits, clause,
   coaction, commitment, committal, compulsatory, compulsion,
   compulsiveness, compulsoriness, compulsory, condition, constraint,
   contingency, contract, courtesy, covenant, deal, debt, dedication,
   deference, demand, devoir, devotion, donnee, due, dues, duress,
   duties and responsibilities, duty, effort, enforcement, engagement,
   enterprise, escalator clause, escape clause, escape hatch, ethics,
   eventuality, favor, fealty, financial commitment, fine print,
   floating debt, forcing, funded debt, given, good deed,
   good offices, good turn, grace, gratefulness, gratitude, grounds,
   homage, imperative, imperious, indebtedness, indebtment,
   inevitability, irresistibility, joker, kicker, kind deed,
   kind offices, kindly act, kindness, labor of love, liability,
   liableness, likelihood, likeliness, limiting condition,
   line of duty, loyalty, mandatedness, mandatoriness, maturity,
   mercy, mission, mitzvah, must, national debt, necessariness,
   necessitation, necessitude, necessity, need, obligatoriness,
   obligatory, obligement, office, onus, operation, ought,
   outstanding debt, parameter, part, place, plan, pledge,
   possibility, preengagement, prerequisite, probability, program,
   project, promise, proneness, proposition, provision, provisions,
   proviso, public debt, recognizance, required, requirement,
   requisite, respect, responsibility, restraint, right,
   saving clause, score, self-imposed duty, sense of obligation,
   service, sine qua non, small print, specification, stipulation,
   string, task, terms, thankfulness, trust, turn, ultimatum,
   uncollectibles, understanding, undertaking, unfulfilled pledge,
   venture, verbal agreement, weakness, whereas, work



Bouvier's Law Dictionary, Revised 6th Ed (1856):

OBLIGATION. In its general and most extensive sense, obligation is
synonymous with duty. In a more technical meaning, it is a tie which binds
us to pay or to do something agreeably to the laws and customs of the
country in which the obligation is made. Just. Inst. 1. 3, t. 14. The term
obligation also signifies the instrument or writing by which the contract is
witnessed. And in another sense, an obligation still subsists, although the
civil obligation is said to be a bond containing a penalty, with a condition
annexed for the payment of money, performance of covenants or the like; it
differs from a bill, which is generally without a penalty or condition,
though it may be obligatory. Co. Litt. 172. It is also defined to be a deed
whereby a man binds himself under a penalty to do a thing. Com. Dig.
Obligation, A. The word obligation, in its most technical signification, ex
vi termini, imports a sealed instrument. 2 S. & R. 502; 6 Vern. 40; 1
Blackf. 241; Harp. R. 434; 2 Porter, 19; 1 Bald. 129. See 1 Bell's Com. b.
3, p. 1, c. 1, page 293; Bouv. Inst. Index, h.t.
     2. Obligations are divided into imperfect obligations, and perfect
obligations.
     3. Imperfect obligations are those which are not binding on us as
between man and man, and for the non-performance of which we are accountable
to God only; such as charity or gratitude. In this sense an obligation is a
mere duty. Poth. Ob. art. Prel. n. 1.
     4. A perfect obligation is one which gives a right to another to
require us to give him something or not to do something. These obligations
are either natural or moral, or they are civil.
     5. A natural or moral obligation is one which cannot be enforced by
action, but which is binding on the party who makes it, in conscience and
according to natural justice. As for instance, when the action is barred by
the act of limitation, a natural obligation is extinguished. 5 Binn. 573.
Although natural obligations cannot be enforced by action, they have the
following effect: 1. No suit will lie to recover back what has been paid, or
given in compliance with a natural obligation. 1 T. R. 285; 1 Dall. 184, 2.
A natural obligation is a sufficient consideration for a new contract. 5
Binn. 33; 2 Binn. 591; Yelv. 41, a, n. 1; Cowp. 290; 2 Bl. Com. 445; 3 B. &
P. 249, n.; 2 East, 506; 3 Taunt. 311; 5 Taunt. 36; Yelv. 41, b. note; 3
Pick. 207 Chit. Contr. 10.
     6. A civil obligation is one which has a binding operation in law,
vinculum juris, and which gives to the obligee the right of enforcing it in
a court of justice; in other words, it is an engagement binding on the
obligor. 12 Wheat. It:. 318, 337; 4 Wheat. R. 197.
     7. Civil obligations are divided into express and implied, pure. and
conditional, primitive and secondary, principal and accessory, absolute and
alternative, determinate and indeterminate, divisible and indivisible,
single and penal, and joint and several. They are also purely personal,
purely real, and both real and mixed at the same time.
     8. Express or conventional obligations are those by which the obligor
binds himself in express terms to perform his obligation.
     9. An implied obligation is one which arises by operation of law; as,
for example, if I send you daily a loaf of bread, without any express
authority, and you make use of it in your family, the law raises an
obligation on your part to pay me the value of the bread.
    10. A pure or simple obligation is one which is not suspended by any
condition, either because it has been contacted without condition, or,
having been contracted with one, it has been fulfilled.
    11. A conditional obligation is one the execution of which is suspended
by a condition which has not been accomplished, and subject to which it has
been contracted.
    12. A primitive obligation, which in one sense may also be called a
principal obligation, is one which is contracted with a design that it
should, itself, be the first fulfilled.
    13. A secondary obligation is one which is contrasted, and is to be
performed, in case the primitive cannot be. For example, if I sell you my
house, I bind myself to give a title, but I find I cannot, as the title is
in another, then my secondary obligation is to pay you damages for my non-
performance of my obligation.
    14. A principal obligation is one which is the most important object of
the engagement of the contracting parties.
    15. An accessory obligation is one which is dependent on the principal
obligation; for example, if I sell you a house and lot of ground, the
principal obligation on my part is to make you a title for it; the accessory
obligation is to deliver you all the title papers which I have relating to
it; to take care of the estate till it is delivered to you, and the like.
    16. An absolute obligation is one which gives no alternative to the
obligor, but he is bound to fulfill it according to his engagement.
    17. An alternative obligation is, where a person engages to do, or to
give several things in such a manner that the payment of one will acquit him
of all; as if A agrees to give B, upon a sufficient consideration, a horse,
or one hundred dollars. Poth. Obl. Pt. 2, c. 3, art. 6, No.. 245.
    18. In order to constitute an alternative obligation, it is necessary
that two or more things should be promised disjunctively; where they are
promised conjunctively, there are as many obligations as the things which
are enumerated, but where they are in the alternative, though they are all
due, there is but one obligation, which may be discharged by the payment of
any of them.
    19. The choice of performing one of the obligations belongs to the
obligor, unless it is expressly agreed that all belong to the creditor.
Dougl. 14; 1 Lord Raym. 279; 4 N. S. 167. If one of the acts is prevented by
the obligee, or the act of God, the obligor is discharged from both. See 2
Evans' Poth. Ob. 52 to 54; Vin. Ab. Condition, S b; and articles
Conjunctive; Disjunctive; Election.
    20. A determinate obligation, is one which has for its object a certain
thing; as an obligation to  deliver a certain horse named Bucephalus. In
this case the obligation can only be discharged by delivering the identical
horse.
    21. An indeterminate obligation is one where the obligor binds himself
to deliver one of a certain species; as, to deliver a horse, the delivery of
any horse will discharge the obligation.
    22. A divisible obligation is one which being a unit may nevertheless be
lawfully divided with or without the consent of the parties. It is clear it
may be divided by consent, as those who made it, may modify or change it as
they please. But some obligations may be divided without the consent of the
obligor; as, where a tenant is bound to pay two hundred dollars a year rent
to his landlord, the obligation is entire, yet, if his landlord dies and
leaves two sons, each will be entitled to one hundred dollars; or if the
landlord sells one undivided half of the estate yielding the rent, the
purchaser will be entitled to receive one hundred dollars, and the seller
the other hundred. See Apportionment.
    23. An indivisible obligation is one which is not susceptible of
division; as, for example, if I promise to pay you one hundred dollars, you
cannot assign one half of this to another, so as to give him a right of
action against me for his share. See Divisible.
    24. A single obligation is one without any penalty; as, where I simply
promise to pay you one hundred dollars. This is called a single bill, when
it is under seal.
    25. A penal obligation is one to which is attached a penal clause which
is to be enforced, if the principal obligation be not performed. In general
equity will relieve against a penalty, on the fulfillment of the principal
obligation. See Liquidated damages; Penalty.
    26. A joint obligation is one by which several obligors promise to the
obligee to perform the obligation. When the obligation is only joint and the
obligors do not promise separately to fulfill their engagement they must be
all sued, if living, to compel the performance; or, if any be dead, the
survivors must all be sued. See Parties to actions.
    27. A several obligation is one by which one individual, or if there be
more, several individuals bind themselves separately to perform the
engagement. In this case each obligor may be sued separately, and if one or
more be dead, their respective executors may be sued. See Parties to
actions.
    28. The obligation is, purely personal when the obligor binds himself to
do a thing; as if I give my note for one thousand dollars, in that case my
person only is bound, for my property is liable for the debt only while it
belongs to me, and, if I lawfully transfer it to a third person, it is
discharged.
    29. The obligation is personal in another sense, as when the obligor
binds himself to do a thing, and he provides his heirs and executors shall
not be bound; as, for example, when he promises to pay a certain sum yearly
during his life, and the payment is to cease at his death.
    30. The obligation is real when real estate, and not the person, is
liable to the obligee for the performance. A familiar example will explain
this: when an estate owes an easement, as a right of way, it is the thing
and not the owner who owes the easement. Another instance occurs when a
person buys an estate which has been mortgaged, subject to the mortgage, he
is not liable for the debt, though his estate is. In these cases the owner
has an interest only because he is seised of the servient estate, or the
mortgaged premises, and he may discharge himself by abandoning or parting
with the property.
    31. The obligation is both personal and real when the obligor has bound
himself, and pledged his estate for the fulfillment of his obligation.