The Collaborative International Dictionary of English v.0.48:

Pegmatite \Peg"ma*tite\, n. [From Gr. ? something fastened
   together, in allusion to the quartz and feldspar in graphic
   granite: cf. F. pegmatite. See Pegm.] (Min.)
   (a) Graphic granite. See under Granite.
   (b) More generally, a coarse granite occurring as vein
       material in other rocks.
       [1913 Webster]

WordNet (r) 3.0 (2006):

pegmatite
    n 1: a form of igneous rock consisting of extremely coarse
         granite resulting from the crystallization of magma rich in
         rare elements