The Collaborative International Dictionary of English v.0.48:

aquifer \aquifer\ n.
   1. an underground bed or layer yielding ground water for
      wells and springs etc.; as, the Oglala aquifer. The water
      contained in an aquifer may be of great age, and in such
      cases is sometimes called

   fossil water.
      [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

aquifer
    n 1: underground bed or layer yielding ground water for wells
         and springs etc