(1):
(n.) The depression formed by the meeting of two slopes on a flat roof.
(2):
(n.) 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.
(3):
(n.) 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.