The Collaborative International Dictionary of English v.0.48:

Pecan \Pe*can"\ (p[-e]*k[a^]n"), n. [Cf. F. pacane the nut.]
   (Bot.)
   A species of hickory (Carya oliv[ae]formis), growing in
   North America, chiefly in the Mississippi valley and in
   Texas, where it is one of the largest of forest trees; also,
   its fruit, a smooth, oblong nut, an inch or an inch and a
   half long, with a thin shell and well-flavored meat. [Written
   also pacane.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

pecan
    n 1: wood of a pecan tree
    2: tree of southern United States and Mexico cultivated for its
       nuts [syn: pecan, pecan tree, Carya illinoensis, Carya
       illinoinsis]
    3: smooth brown oval nut of south central United States