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]