stoma

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Botany One of the minute pores in the epidermis of a leaf or stem through which gases and water vapor pass.
• noun Anatomy A small aperture in the surface of a membrane.
• noun A surgically constructed opening, especially one in the abdominal wall that permits the passage of waste after a colostomy or ileostomy.
• noun Zoology A mouthlike opening, such as the oral cavity of a nematode.

from The Century Dictionary.

• noun In zoology, a mouth or ingestive opening; an oral orifice; an ostium or ostiole: chiefly used of small or simple apertures, as a cytostome; hence, also, a small opening of any kind through which something may pass in or out; a pore.
• noun In botany, a minute orifice or slit in the epidermis of leaves, etc., which opens directly into air-cavities or intercellular spaces that pervade the interior, and through which free ingress and egress of air take place; a breathing-pore.
• noun In Sweden borg's philosophy, a cubical figure with hollowed surfaces, being the figure of the interstices of spheres arranged in what Swedenborg calls the fixed quadrilateral pyramidal position, supposed to be that natural to the spherical particles of water.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) One of the minute apertures between the cells in many serous membranes.
• noun The minute breathing pores of leaves or other organs opening into the intercellular spaces, and usually bordered by two contractile cells.
• noun The line of dehiscence of the sporangium of a fern. It is usually marked by two transversely elongated cells. See Illust. of Sporangium.
• noun (Zoöl.) A stigma. See Stigma, n., 6 (a) & (b).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany One of the tiny pores in the epidermis of a leaf or stem through which gases and water vapor pass.
• noun A small opening in a membrane; a surgically constructed opening, especially one in the abdominal wall that permits the passage of waste after a colostomy or ileostomy.
• noun zoology A mouthlike opening, such as the oral cavity of a nematode.
• noun An artificial anus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a minute epidermal pore in a leaf or stem through which gases and water vapor can pass
• noun a mouth or mouthlike opening (especially one created by surgery on the surface of the body to create an opening to an internal organ)