From The Collaborative International Dictionary of English v.0.48 [gcide]:
1. 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.
2. (Arch.) (a) The place of meeting of two slopes of a roof, which have their plates running in different directions, and form on the plan a reentrant angle. (b) The depression formed by the meeting of two slopes on a flat roof.
From WordNet (r) 3.0 (2006) [wn]:
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