tractor trailer vs semi

tractor trailer and semi both are nouns.

Word	Noun	Adjective	Verb	Adverb
tractor trailer	Yes	No	No	No
semi	Yes	No	No	No

As nouns, tractor trailer and semi are synonyms defined as:

tractor trailer and semi: a truck consisting of a tractor and trailer together

Other synonyms of tractor trailer include articulated lorry, rig, trailer truck, trucking rig.

tractor trailer (noun)	semi (noun)
a truck consisting of a tractor and trailer together	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