semi vs tandem trailer

semi and tandem trailer both are nouns.

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

As nouns, tandem trailer is a hyponym of semi; that is, tandem trailer is a word with a more specific, narrower meaning than semi:

semi (noun)	tandem trailer (noun)
a trailer having wheels only in the rear; the front is supported by the towing vehicle	trucking rig with two trailers in tandem
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