Real Dictionary

lilac
variable noun: A lilac or a lilac tree is a small tree which has sweet-smelling purple, pink, or white flowers in large, cone-shaped groups.

lilac in American English
any of a genus (Syringa) of hardy shrubs or trees of the olive family, with large clusters of tiny, fragrant flowers ranging in color from white, through many shades of lavender, to deep crimson
of a pale-purple color
adjective: having the color lilac
noun: any of various shrubs belonging to the genus Syringa, of the olive family, as S. vulgaris, having large clusters of fragrant purple or white flowers: the state flower of New Hampshire

lilac in British English
noun: any of various Eurasian oleaceous shrubs or small trees of the genus Syringa, esp S. vulgaris ( common lilac) which has large sprays of purple or white fragrant flowers