The Collaborative International Dictionary of English v.0.48:

Amaranthus \Am`a*ran"thus\ ([a^]m`[.a]*r[a^]n"th[u^]s),
Amarantus \Am`a*ran"tus\ ([a^]m`[.a]*r[a^]n"t[u^]s), n.
   Same as Amaranth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Amaranthus
    n 1: large widely distributed genus of chiefly coarse annual
         herbs [syn: Amaranthus, genus Amaranthus]