noun pl. valleys -·leys

1. a stretch of low land lying between hills or mountains and usually having a river or stream flowing through it
2. the land drained or watered by a great river system the Mississippi valley
3. any long dip or hollow, as the trough of a wave
4. Archit. the trough formed where two slopes of a roof meet, or where the roof meets a wall

Etymology: ME valey < OFr valee < val < L vallis, vale < IE base *wel-, to turn, roll > walk, well