The Collaborative International Dictionary of English v.0.48:

Sumac \Su"mac\, Sumach \Su"mach\, n. [F. sumac, formerly sumach
   (cf. Sp. zumaque), fr. Ar. summ[=a]q.] [Written also
   shumac.]
   1. (Bot.) Any plant of the genus Rhus, shrubs or small
      trees with usually compound leaves and clusters of small
      flowers. Some of the species are used in tanning, some in
      dyeing, and some in medicine. One, the Japanese Rhus
      vernicifera, yields the celebrated Japan varnish, or
      lacquer.
      [1913 Webster]

   2. The powdered leaves, peduncles, and young branches of
      certain species of the sumac plant, used in tanning and
      dyeing.
      [1913 Webster]

   Poison sumac. (Bot.) See under Poison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sumac
    n 1: wood of a sumac
    2: a shrub or tree of the genus Rhus (usually limited to the
       non-poisonous members of the genus) [syn: sumac, sumach,
       shumac]