backcross vs crossbreed

backcross is not a noun while crossbreed is a noun.

Word	Noun	Adjective	Verb	Adverb
backcross	No	No	Yes	No
crossbreed	Yes	No	Yes	No

As verbs, crossbreed is a hypernym of backcross; that is, crossbreed is a word with a broader meaning than backcross:

backcross: mate a hybrid of the first generation with one of its parents
crossbreed: breed animals or plants using parents of different races and varieties

Other hypernyms of backcross include cross, hybridise, hybridize, interbreed.

backcross (noun)	crossbreed (noun)
	(genetics) an organism that is the offspring of genetically dissimilar parents or stock; especially offspring produced by breeding plants or animals of different varieties or breeds or species

backcross (verb)	crossbreed (verb)
mate a hybrid of the first generation with one of its parents	breed animals or plants using parents of different races and varieties