In plumbing, a trap, 1 whose seal (usually between 3 and 4 in.; approx. 7.5 and 10 cm) is not easily broken; the diameter is not greater than 4 in. (10 cm), and the water held in the trap is not less than 1 qt (0.95 liter).
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.