valley

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An elongated lowland between ranges of mountains, hills, or other uplands, often having a river or stream running along the bottom.
• noun An extensive area of land drained or irrigated by a river system.
• noun A depression or hollow resembling or suggesting a valley, as the point at which the two slopes of a roof meet.

from The Century Dictionary.

• noun A depression, or a relatively low and somewhat level area, more or less completely inclosed by hills or mountains; the basin of a stream of any size, or the area drained by it, and, in accordance with more general usage, the part of that area which lies near the stream and is not much raised above its level.
• noun Hence, any similar depression of any size.
• noun Specifically, in architecture, the internal angle formed by the meeting of two inclined sides of a roof.
• noun The depression between two ridges of a tooth, typically shown in such molars as those of the tapir and mastodon: correlated with lake, which is a depression surrounded by a raised ridge of enamel such as occurs in the molars of a horse.

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

• noun The space inclosed between ranges of hills or mountains; the strip of land at the bottom of the depressions intersecting a country, including usually the bed of a stream, with frequently broad alluvial plains on one or both sides of the stream. Also used figuratively.
• noun The place of meeting of two slopes of a roof, which have their plates running in different directions, and form on the plan a reëntrant angle.
• noun The depression formed by the meeting of two slopes on a flat roof.
• noun (Arch.) a board for the reception of the lead gutter in the valley of a roof. The valley board and lead gutter are not usual in the United States.
• noun (Arch.) the rafter which supports the valley.
• noun (Arch.) a roof having one or more valleys. See Valley, 2, above.

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

• noun An elongated depression between hills or mountains, often with a river flowing through it.
• noun The area which drains into a river.
• noun Any structure resembling one, e.g., the meeting point of two pitched roofs.
• noun The internal angle formed by the intersection of two sloping roof planes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a long depression in the surface of the land that usually contains a river