semitrailer vs semi

semitrailer and semi both are nouns.

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

As nouns, semitrailer and semi are synonyms defined as:

semitrailer and semi: a trailer having wheels only in the rear; the front is supported by the towing vehicle

semitrailer (noun)	semi (noun)
a trailer having wheels only in the rear; the front is supported by the towing vehicle	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