The Collaborative International Dictionary of English v.0.48:

Occupancy \Oc"cu*pan*cy\, n. [See Occupant.]
   1. The act of taking or holding possession, especially of
      real property or rental property; possession; occupation.
      [1913 Webster]

   2. The state or condition of being occupied; as, occupancy by
      more than 250 people is dangerous and unlawful.
      [PJC]

   3. The period of time during which one occupies a property.
      [PJC]

   Title by occupancy (Law), a right of property acquired by
      taking the first possession of a thing, or possession of a
      thing which belonged to nobody, and appropriating it.
      --Blackstone. Kent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

occupancy
    n 1: an act of being a tenant or occupant [syn: occupancy,
         tenancy]
    2: 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]