The Collaborative International Dictionary of English v.0.48:

Amniota \Am`ni*o"ta\, n. pl. [NL. See Amnion.] (Zool.)
   That group of vertebrates which develops in its embryonic
   life the envelope called the amnion. It comprises the
   reptiles, the birds, and the mammals.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Amniota
    n 1: higher vertebrates (reptiles, birds and mammals) possessing
         an amnion during development