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"