The Collaborative International Dictionary of English v.0.48:

Valley \Val"ley\, n.; pl. Valleys. [OE. vale, valeie, OF.
   val['e]e, valede, F. vall['e]e, LL. vallata, L. vallis,
   valles. See Vale.]
   [1913 Webster]
   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.
      [1913 Webster]

            The valley of the shadow of death.    --Ps. xxiii.
                                                  4.
      [1913 Webster]

            Sweet interchange
            Of hill and valley, rivers, woods, and plains.
                                                  --Milton.
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   Note: Deep and narrow valleys with abrupt sides are usually
         the results of erosion by water, and are called
         gorges, ravines, canyons, gulches, etc.
         [1913 Webster]

   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.
          [1913 Webster]

   Valley board (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.

   Valley rafter, or Valley piece (Arch.), the rafter which
      supports the valley.

   Valley roof (Arch.), a roof having one or more valleys. See
      Valley, 2, above.
      [1913 Webster]