The Collaborative International Dictionary of English v.0.48:

Allegiance \Al*le"giance\, n. [OE. alegeaunce; pref. a- + OF.
   lige, liege. The meaning was influenced by L. ligare to bind,
   and even by lex, legis, law. See Liege, Ligeance.]
   1. The tie or obligation, implied or expressed, which a
      subject owes to his sovereign or government; the duty of
      fidelity to one's king, government, or state.
      [1913 Webster]

   2. Devotion; loyalty; as, allegiance to science.
      [1913 Webster]

   Syn: Loyalty; fealty.

   Usage: Allegiance, Loyalty. These words agree in
          expressing the general idea of fidelity and attachment
          to the "powers that be." Allegiance is an obligation
          to a ruling power. Loyalty is a feeling or sentiment
          towards such power. Allegiance may exist under any
          form of government, and, in a republic, we generally
          speak of allegiance to the government, to the state,
          etc. In well conducted monarchies, loyalty is a
          warm-hearted feeling of fidelity and obedience to the
          sovereign. It is personal in its nature; and hence we
          speak of the loyalty of a wife to her husband, not of
          her allegiance. In cases where we personify, loyalty
          is more commonly the word used; as, loyalty to the
          constitution; loyalty to the cause of virtue; loyalty
          to truth and religion, etc.
          [1913 Webster]

                Hear me, recreant, on thine allegiance hear me!
                                                  --Shak.
          [1913 Webster]

                So spake the Seraph Abdiel, faithful found, . .
                .
                Unshaken, unseduced, unterrified,
                His loyalty he kept, his love, his zeal.
                                                  --Milton.
          [1913 Webster]

WordNet (r) 3.0 (2006):

allegiance
    n 1: the act of binding yourself (intellectually or emotionally)
         to a course of action; "his long commitment to public
         service"; "they felt no loyalty to a losing team" [syn:
         commitment, allegiance, loyalty, dedication]
    2: the loyalty that citizens owe to their country (or subjects
       to their sovereign) [syn: allegiance, fealty]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "allegiance":
   acquiescence, adherence, adhesion, assigned task, attachment,
   bona fides, bond, bonne foi, bounden duty, burden, business,
   call of duty, charge, commitment, compliance, conformity,
   consecration, constancy, dedication, deference, devoir,
   devotedness, devotion, duteousness, duties and responsibilities,
   dutifulness, duty, ethics, faith, faithfulness, fealty, fidelity,
   firmness, good faith, homage, honor, imperative, line of duty,
   loyalty, mission, must, obedience, obediency, obligation,
   observance, onus, ought, piety, place, respect, self-imposed duty,
   service, servility, servitium, staunchness, steadfastness,
   submission, submissiveness, suit and service, suit service, tie,
   troth, true blue, trueness, willingness



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

ALLEGIANCE. The tie which binds the citizen to the government, in return for
the protection which the government affords him.
     2. It is natural, acquired, or local. Natural allegiance is such as is
due from all men born within the United States; acquired allegiance is that
which is due by a naturalized citizen. It has never been decided whether a
citizen can, by expatriation, divest himself absolutely of that character. 2
Cranch, 64; 1 Peters' C. C. Rep. 159; 7 Wheat. R. 283; 9 Mass. R. 461.
Infants cannot assume allegiance, (4 Bin. 49) although they enlist in the
army of the United States. 5 Bin. 429.
     3. It seems, however, that he cannot renounce his allegiance to the
United States without the permission of the government, to be declared by
law. But for commercial purposes he may acquire the rights of a citizen of
another country, and the place of his domicil determines the character of a
party as to trade. 1 Kent, Com. 71; Com. Rep. 677; 2 Kent, Com. 42.
     4. Local allegiance is that which is due from an alien, while resident
in the United States, for the protection which the government affords him. 1
Bl. Com. 366, 372; Com. Dig. h.t; Dane's Ab. Index, h.t.; 1 East, P.C. 49
to 57.



The Devil's Dictionary (1881-1906):

ALLEGIANCE, n.

    This thing Allegiance, as I suppose,
    Is a ring fitted in the subject's nose,
    Whereby that organ is kept rightly pointed
    To smell the sweetness of the Lord's anointed.
                                                                  G.J.