coax cable vs transmission line

coax cable and transmission line both are nouns.

Word	Noun	Adjective	Verb	Adverb
coax cable	Yes	No	No	No
transmission line	Yes	No	No	No

As nouns, transmission line is a hypernym of coax cable; that is, transmission line is a word with a broader meaning than coax cable:

coax cable: a transmission line for high-frequency signals
transmission line: a conductor for transmitting electrical or optical signals or electric power

Other hypernyms of coax cable include cable, line.

coax cable (noun)	transmission line (noun)
a transmission line for high-frequency signals	a conductor for transmitting electrical or optical signals or electric power