The Collaborative International Dictionary of English v.0.48:

Aquarium \A*qua"ri*um\, n.; pl. E. Aquariums, L. Aquaria.
   [L. See Aquarius, Ewer.]
   An artificial pond, or a globe or tank (usually with glass
   sides), in which living specimens of aquatic animals or
   plants are kept.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aquarium
    n 1: a tank or pool or bowl filled with water for keeping live
         fish and underwater animals [syn: aquarium, fish tank,
         marine museum]