dam vs weir

dam and weir both are nouns.

Word	Noun	Adjective	Verb	Adverb
dam	Yes	No	Yes	No
weir	Yes	No	No	No

As nouns, weir is a hyponym of dam; that is, weir is a word with a more specific, narrower meaning than dam:

dam: a barrier constructed to contain the flow of water or to keep out the sea
weir: a low dam built across a stream to raise its level or divert its flow

Other hyponyms of dam include milldam.

dam (noun)	weir (noun)
female parent of an animal especially domestic livestock	a low dam built across a stream to raise its level or divert its flow
a barrier constructed to contain the flow of water or to keep out the sea	a fence or wattle built across a stream to catch or retain fish
a metric unit of length equal to ten meters

dam (verb)	weir (verb)
obstruct with, or as if with, a dam