The Collaborative International Dictionary of English v.0.48:

Vesica \Ve*si"ca\, n. [L.]
   A bladder.
   [1913 Webster]

   Vesica piscis. [L., fish bladder.] (Eccl. Art) A glory, or
      aureole, of oval shape, or composed of two arcs of circles
      usually represented as surrounding a divine personage.
      More rarely, an oval composed of two arcs not representing
      a glory; a solid oval, etc.
      [1913 Webster]