Stoma

Definition
noun, plural: stomata
(botany) A tiny pore in a plant leaf surrounded by a pair of guard cells that regulate its opening and closure, and serves as the site for gas exchange.
(zoology) Mouth-like opening, such as the stoma (or the oral cavities) of nematodes.
(anatomy) A natural opening in the body, such as the mouth.
(medicine) Artificial opening in the body created by surgery, connecting a portion of the body cavity to the outside environment.
Supplement
Stomata may refer to the natural openings to the outside environment, such as those on plant leaves or oral cavities of certain animals. Stomata may also refer to the artificial body openings created by surgery.
In plants, the stomata are actually the pores created by the swelling of guard cells to allow CO₂ to enter into the leaf, which is a necessary reactant of photosynthesis. The water vapor and O₂ are also allowed to escape via the pore. In order to form a pore or stoma, osmotic pressure draws water to increase the cells volume; this in turn causes the guard cells to bow apart from each other because the inner wall of the pore is more rigid than the wall on the oppostie side of the cell.
Stomata are present in all terrestrial plants (in sporophyte phase), except for the liverworts. Dicots usually have more stomata on the lower epidermis than the upper epidermis whereas monocots usually have the same number of stomata on both sides. Plants whose leaves float in water have stomata only on the upper epidermis whereas plants whose leaves are completely submerged may lack stomata entirely.
In medicine, any hollow organ can be manipulated into an artificial stoma as necessary. This includes the esophagus, stomach, duodenum, ileum, colon, pleural cavity, ureters, etc. A widely known example is colostomy, which is a stoma created in the abdominal wall to permit the passage of waste.
Word origin: From Ancient Greek stoma, “‘mouth’”.
Related forms: stomal, stomatal (adjective).
Synonym: stomate (botany).