The Collaborative International Dictionary of English v.0.48:

Occupation \Oc`cu*pa"tion\, n. [L. occupatio: cf. F.
   occupation.]
   1. The act or process of occupying or taking possession;
      actual possession and control; the state of being
      occupied; a holding or keeping; tenure; use; as, the
      occupation of lands by a tenant.
      [1913 Webster]

   2. That which occupies or engages the time and attention.
      [1913 Webster +PJC]

   3. Specfically: The principal business of one's life; the
      principal work by which one earns one's livelihood;
      vocation; employment; profession; calling; trade;
      avocation; as, these days many people continue to practice
      their occupation well into their seventies.
      [1913 Webster +PJC]

            Absence of occupation is not rest.    --Cowper.
      [1913 Webster]

   Occupation bridge (Engin.), a bridge connecting the parts
      of an estate separated by a railroad, a canal, or an
      ordinary road.
      [1913 Webster]

   Syn: Occupancy; possession; tenure; use; employment;
        avocation; engagement; vocation; calling; office; trade;
        profession.
        [1913 Webster]

WordNet (r) 3.0 (2006):

occupation
    n 1: the principal activity in your life that you do to earn
         money; "he's not in my line of business" [syn:
         occupation, business, job, line of work, line]
    2: the control of a country by military forces of a foreign
       power [syn: occupation, military control]
    3: any activity that occupies a person's attention; "he missed
       the bell in his occupation with the computer game"
    4: the act of occupying or taking possession of a building;
       "occupation of a building without a certificate of occupancy
       is illegal" [syn: occupation, occupancy, moving in]
    5: the period of time during which a place or position or nation
       is occupied; "during the German occupation of Paris"

Moby Thesaurus II by Grady Ward, 1.0:

188 Moby Thesaurus words for "occupation":
   abiding, abode, act, acting, action, activism, activities,
   activity, adoption, adverse possession, affair, affairs, agency,
   alodium, appointment, appropriation, arrogation, art, assumption,
   bag, behavior, bondage, burgage, business, calling, career,
   career building, careerism, claim, cohabitation, colonization,
   colony, commerce, commorancy, concern, concernment, conduct,
   conquest, control, craft, de facto, de jure, dependency,
   derivative title, direction, doing, driving, dwelling, employ,
   employment, enslavement, enterprise, execution, exercise, fee fief,
   fee position, fee simple, fee simple absolute,
   fee simple conditional, fee simple defeasible,
   fee simple determinable, fee tail, feodum, feud, fiefdom, field,
   frankalmoign, free socage, freehold, function, functioning, game,
   gavelkind, habitancy, habitation, handicraft, handling,
   having title to, hold, holding, indent, inhabitancy, inhabitation,
   inhabiting, interest, job, knight service, labor, lay fee, lease,
   leasehold, legal claim, legal possession, lifework, line,
   line of business, line of work, living, lodging, lookout,
   management, mandate, manipulation, matter, metier, mission,
   movements, mystery, nesting, number, occupancy, operancy,
   operation, operations, oppression, original title, owning,
   performance, performing, play, position, possessing, possession,
   post, practice, praxis, preemption, preoccupancy, preoccupation,
   prepossession, prescription, profession, property, property rights,
   proprietary rights, pursuit, racket, requisition, residence,
   residency, residing, responsibility, rule, running, seisin,
   seizure, service, settlement, situation, skill, socage, sojourning,
   specialization, specialty, squatting, staying, staying over,
   steering, stopping, subjection, subjugation, sublease, suzerainty,
   swing, take-over, takeover, taking over, tenancy, tenantry, tenure,
   tenure in chivalry, thing, title, trade, underlease, undertaking,
   undertenancy, usucapion, usurpation, villein socage, villeinhold,
   villenage, vocation, walk, walk of life, work, working, workings



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

OCCUPATION. Use or tenure; as, the house is in the occupation of A B. A
trade, business or mystery; as the occupation of a printer. Occupancy.
(q.v.)
     2. In another sense occupation signifies a putting out of a man's
freehold in time of war. Co. Litt. s. 412. See Dependency; Possession.