The Collaborative International Dictionary of English v.0.48:

Weir \Weir\ (w[=e]r), Wear \Wear\,n. [OE. wer, AS. wer; akin to
   G. wehr, AS. werian to defend, protect, hinder, G. wehren,
   Goth. warjan; and perhaps to E. wary; or cf. Skr. v[.r] to
   check, hinder. [root]142. Cf. Garret.]
   1. A dam in a river to stop and raise the water, for the
      purpose of conducting it to a mill, forming a fish pond,
      or the like.
      [1913 Webster]

   2. A fence of stakes, brushwood, or the like, set in a
      stream, tideway, or inlet of the sea, for taking fish.
      [1913 Webster]

   3. A long notch with a horizontal edge, as in the top of a
      vertical plate or plank, through which water flows, --
      used in measuring the quantity of flowing water.
      [1913 Webster]

WordNet (r) 3.0 (2006):

weir
    n 1: a low dam built across a stream to raise its level or
         divert its flow
    2: a fence or wattle built across a stream to catch or retain
       fish