Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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