The Collaborative International Dictionary of English v.0.48:

Vesicle \Ves"i*cle\, n. [L. vesicula, dim. of vesica a bladder,
   blister; akin to Skr. vasti bladder: cf. F. v['e]sicule.]
   A bladderlike vessel; a membranous cavity; a cyst; a cell.
   Specifically:
   [1913 Webster]
   (a) (Bot.) A small bladderlike body in the substance of a
       vegetable, or upon the surface of a leaf.
       [1913 Webster]
   (b) (Med.) A small, and more or less circular, elevation of
       the cuticle, containing a clear watery fluid.
       [1913 Webster]
   (c) (Anat.) A cavity or sac, especially one filled with
       fluid; as, the umbilical vesicle.
       [1913 Webster]
   (d) (Zool.) A small convex hollow prominence on the surface
       of a shell or a coral.
       [1913 Webster]
   (e) (Geol.) A small cavity, nearly spherical in form, and
       usually of the size of a pea or smaller, such as are
       common in some volcanic rocks. They are produced by the
       liberation of watery vapor in the molten mass.
       [1913 Webster]

WordNet (r) 3.0 (2006):

vesicle
    n 1: a small anatomically normal sac or bladderlike structure
         (especially one containing fluid) [syn: vesicle, cyst]

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "vesicle":
   air bubble, balloon, bilge, birthmark, blackhead, bladder, blain,
   bleb, blemish, blister, blob, blood blister, boss, bow, bubble,
   bulb, bulge, bulla, bump, bunch, burl, button, cahot, check, chine,
   cicatrix, clump, comedo, condyle, convex, crack, crater, craze,
   defacement, defect, deformation, deformity, disfiguration,
   disfigurement, distortion, dowel, ear, fault, fever blister,
   flange, flap, flaw, freckle, gall, globule, gnarl, handle,
   hemangioma, hickey, hill, hump, hunch, jog, joggle, keloid, kink,
   knob, knot, knur, knurl, lentigo, lip, loop, lump, milium, mole,
   mountain, needle scar, nevus, nub, nubbin, nubble, papilloma, peg,
   pimple, pit, pock, pockmark, port-wine mark, port-wine stain,
   pustule, rib, ridge, rift, ring, scab, scar, scratch,
   sebaceous cyst, shoulder, soap bubble, spine, split,
   strawberry mark, stud, sty, style, tab, track, tubercle, tubercule,
   twist, verruca, wale, warp, wart, weal, welt, wen, whitehead