second hand vs mediator

second hand and mediator both are nouns.

Word	Noun	Adjective	Verb	Adverb
second hand	Yes	No	No	Yes
mediator	Yes	No	No	No

As nouns, mediator is a hypernym of second hand; that is, mediator is a word with a broader meaning than second hand:

Other hypernyms of second hand include go-between, intercessor, intermediary, intermediator.

second hand (noun)	mediator (noun)
hand marking seconds on a timepiece	a negotiator who acts as a link between parties
an intermediate person; used in the phrase `at second hand'

second hand (adverb)	mediator (adverb)
from a source of previously owned goods