The Collaborative International Dictionary of English v.0.48:

Pleistocene \Pleis"to*cene\, a. [Gr. ? most + ? new.] (Geol.)
   Of or pertaining to the epoch, or the deposits, following the
   Tertiary, and immediately preceding man. -- n. The
   Pleistocene epoch, or deposits.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Pleistocene
    n 1: from two million to 11 thousand years ago; extensive
         glaciation of the northern hemisphere; the time of human
         evolution [syn: Pleistocene, Pleistocene epoch,
         Glacial epoch]