Source : Webster's Revised Unabridged Dictionary (1913)

Pustule \Pus"tule\ (?; 135), n. [L. pustula, and pusula: cf. F.
   pustule.] (Med.)
   A vesicle or an elevation of the cuticle with an inflamed
   base, containing pus.

   {Malignant pustule}. See under {Malignant}.

Source : WordNet®

pustule
     n : a small inflamed elevation of skin containing pus; a blister
         filled with pus