draintrap

from The Century Dictionary.

• noun A contrivance to prevent the escape of foul air from drains, while allowing the passage of water into them.

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

• noun See 4th trap, 5.

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

• noun 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.