The Collaborative International Dictionary of English v.0.48:

Stoma \Sto"ma\, n.; pl. Stomata. [NL., fr. Gr. ?, ?, a mouth.]
   1. (Anat.) One of the minute apertures between the cells in
      many serous membranes.
      [1913 Webster]

   2. (Bot.)
      (a) The minute breathing pores of leaves or other organs
          opening into the intercellular spaces, and usually
          bordered by two contractile cells.
      (b) The line of dehiscence of the sporangium of a fern. It
          is usually marked by two transversely elongated cells.
          See Illust. of Sporangium.
          [1913 Webster]

   3. (Zool.) A stigma. See Stigma, n., 6
      (a) &
      (b) .
          [1913 Webster]

WordNet (r) 3.0 (2006):

stoma
    n 1: a minute epidermal pore in a leaf or stem through which
         gases and water vapor can pass [syn: stoma, stomate,
         pore]
    2: a mouth or mouthlike opening (especially one created by
       surgery on the surface of the body to create an opening to an
       internal organ)