weed killer vs weedkiller

weed killer and weedkiller both are nouns.

Word	Noun	Adjective	Verb	Adverb
weed killer	Yes	No	No	No
weedkiller	Yes	No	No	No

As nouns, weed killer and weedkiller are synonyms defined as:

weed killer and weedkiller: a chemical agent that destroys plants or inhibits their growth

Other synonyms of weed killer include herbicide.

weed killer (noun)	weedkiller (noun)
a chemical agent that destroys plants or inhibits their growth	a chemical agent that destroys plants or inhibits their growth