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.
      [1913 Webster]

   2. A light purplish color like that of the flower of the
      purplish lilac.
      [1913 Webster]

   California lilac (Bot.), a low shrub with dense clusters of
      purplish flowers (Ceanothus thyrsiflorus).
      [1913 Webster]

WordNet (r) 3.0 (2006):

lilac
    adj 1: of a pale purple color [syn: lavender, lilac, lilac-
           colored]
    n 1: any of various plants of the genus Syringa having large
         panicles of usually fragrant flowers