From 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]
From 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]