The Collaborative International Dictionary of English v.0.48:

impala \im*pa"la\, n.
   An antelope (Aepyceros melampus) of Southeastern Africa,
   the male of which has ringed lyre-shaped horns, which curve
   first backward, then sideways, then upwards. ALso called
   impalla and pallah.
   [PJC]

WordNet (r) 3.0 (2006):

impala
    n 1: African antelope with ridged curved horns; moves with
         enormous leaps [syn: impala, Aepyceros melampus]