semifinal vs semi

semifinal and semi both are nouns.

Word	Noun	Adjective	Verb	Adverb
semifinal	Yes	No	No	No
semi	Yes	No	No	No

As nouns, semifinal and semi are synonyms defined as:

semifinal and semi: one of the two competitions in the next to the last round of an elimination tournament

semifinal (noun)	semi (noun)
one of the two competitions in the next to the last round of an elimination tournament	a trailer having wheels only in the rear; the front is supported by the towing vehicle
	a truck consisting of a tractor and trailer together
	one of the two competitions in the next to the last round of an elimination tournament