tandem trailer vs semi

tandem trailer and semi both are nouns.

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

As nouns, semi is a hypernym of tandem trailer; that is, semi is a word with a broader meaning than tandem trailer:

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

tandem trailer (noun)	semi (noun)
trucking rig with two trailers in tandem	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