The Collaborative International Dictionary of English v.0.48:

Trillium \Tril"li*um\, n. [NL.; cf. L. trilix triple-woven,
   triple.] (Bot.)
   A genus of liliaceous plants; the three-leaved nightshade; --
   so called because all the parts of the plant are in threes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trillium
    n 1: any liliaceous plant of the genus Trillium having a whorl
         of three leaves at the top of the stem with a single three-
         petaled flower [syn: trillium, wood lily, wake-robin]