Trough, n.
(trawf, 21), n. [A-S. trog, troh, Ger., D., & Icel. trog, Sw. tråg, Dan. trug, It. truogo, O. Fr. troc. Cf. TRUG and TRAY.]
Sense 1. A long, hollow vessel, generally for holding water or other liquid, especially one formed by excavating a log longitudinally on one side; a long tray; also, a wooden channel for conveying water, as to a mill-wheel.
Sense 2. Hence, a channel, receptacle, or depression, of a long and narrow shape; as, a trough between two ridges, and the like.
Trough of the sea, the space between two high waves.