The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Malignant pustule. See under Malignant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pustule
    n 1: a small inflamed elevation of skin containing pus; a
         blister filled with pus

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "pustule":
   abscess, aposteme, bed sore, birthmark, blackhead, blain, bleb,
   blemish, blister, boil, bubo, bulla, bump, bunion, canker,
   canker sore, carbuncle, chancre, chancroid, check, chilblain,
   cicatrix, cold sore, comedo, corn, crack, crater, craze, cyst,
   defacement, defect, deformation, deformity, dilatation, dilation,
   disfiguration, disfigurement, distension, distortion, edema,
   eschar, fault, felon, fester, festering, fever blister, fistula,
   flaw, freckle, furuncle, furunculus, gathering, gumboil,
   hemangioma, hemorrhoids, hickey, intumescence, keloid, kibe, kink,
   lentigo, lesion, lump, milium, mole, needle scar, nevus, papula,
   papule, paronychia, parulis, petechia, piles, pimple, pit, pock,
   pockmark, polyp, port-wine mark, port-wine stain, rift, rising,
   scab, scar, scratch, sebaceous cyst, soft chancre, sore, split,
   stigma, strawberry mark, sty, suppuration, swell, swelling,
   swollenness, track, tubercle, tumefaction, tumescence, tumidity,
   tumor, turgescence, turgescency, turgidity, twist, ulcer,
   ulceration, verruca, vesicle, wale, warp, wart, weal, welt, wen,
   wheal, whelk, whitehead, whitlow, wound