Real Dictionary

weir
countable noun: A weir is a low barrier which is built across a river in order to control or direct the flow of water.
countable noun: A weir is a wooden fence which is built across a stream in order to create a pool for catching fish.

weir in American English
a low dam built in a river to back up or divert water, as for a mill; milldam
noun: a small dam in a river or stream

weir in British English
noun: a low dam that is built across a river to raise the water level, divert the water, or control its flow
noun: David (Russell). born 1979, English wheelchair athlete; won ten medals for Britain, including six golds, over three Olympic Games (2004–2012); won the London Marathon a record eight times

weir in Chemical Engineering 1
noun: A weir is a channel at the entry to or exit from a distillation tray.

weir in Chemical Engineering 2
noun: A weir is a device for measuring flow in an open channel flow.