U-bend
noun
a U-shaped bend in a pipe or drain that traps water in the lower part of the U and prevents the escape of noxious fumes or vapours; trap
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012