The Collaborative International Dictionary of English v.0.48:
Lilac \Li"lac\ (l[imac]"lak), n. [Also lilach.] [Sp. lilac,
lila, Ar. l[imac]lak, fr. Per. l[imac]laj, l[imac]lanj,
l[imac]lang, n[imac]laj, n[imac]l, the indigo plant, or from
the kindred l[imac]lak bluish, the flowers being named from
the color. Cf. Anil.]
1. (Bot.) A shrub of the genus Syringa. There are six
species, natives of Europe and Asia. Syringa vulgaris,
the common lilac, and Syringa Persica, the Persian
lilac, are frequently cultivated for the fragrance and
beauty of their purplish or white flowers. In the British
colonies various other shrubs have this name.
2. A light purplish color like that of the flower of the
California lilac (Bot.), a low shrub with dense clusters of
purplish flowers (Ceanothus thyrsiflorus).