n. A bend, sag, or other device in a waste-pipe arranged so that the liquid contents form a seal which prevents the escape of noxious gases, but permits the flow of liquids.
the GNU version of the Collaborative International Dictionary of English
n. See 4th trap, 5.
The Century Dictionary and Cyclopedia
n. A contrivance to prevent the escape of foul air from drains, while allowing the passage of water into them.