trillium or wake-robin, any plant of the large genus Trillium, attractive spring wildflowers of the family Liliaceae (lily family), native to North America and E Asia. The leaves, petals, and sepals are characteristically in threes, and the single flower may be white, pink, dark red, yellow, or green. The plants have a perennial rootstock that in T. erectum (also called birthroot) was used medicinally by both Native Americans and early colonists. Trillium is classified in the division Magnoliophyta, class Liliopsida, order Liliales, family Liliaceae.