TRILLIUM
trillium, wood lily, wake-robin
(noun) 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
Source: WordNet® 3.1
Etymology
From the genus name.
Noun
trillium (plural trilliums)
Any of several perennial flowering plants, of the genus Trillium, having flowers with three petals
Source: Wiktionary
Tril"li*um, n. Etym: [NL.; cf. L. trilix triple-woven, triple.]
(Bot.)
Definition: A genus of liliaceous plants; the three-leaved nightshade; --
so called because all the parts of the plant are in threes.
Source: Webster’s Unabridged Dictionary 1913 Edition