a low dam built across a stream to raise its level or divert its flow
"We had to climb over an old weir on the riverbank to get to our boat launching site."
a fence or wattle built across a stream to catch or retain fish
"The old weir still spans the narrow river, trapping salmon in the pool below before they can escape downstream."
An adjustable dam placed across a river to regulate the flow of water downstream.
"The local farmers gathered at the weir to adjust its gates and ensure enough irrigation water reached their fields during the drought."
A surname.
"The local history club is hosting a lecture next week on the life of Sir John Weir, who served as the mayor in the early 1900s."