wait vs hold on

wait is a noun but hold on is not a noun.

Word	Noun	Adjective	Verb	Adverb
wait	Yes	No	Yes	No
hold on	No	No	Yes	No

As verbs, hold on is a hyponym of wait; that is, hold on is a word with a more specific, narrower meaning than wait:

Other hyponyms of wait include expect, look forward, look to, hang on, hold the line, anticipate, look for, look to.

wait (noun)	hold on (noun)
the act of waiting (remaining inactive in one place while expecting something)
time during which some action is awaited

wait (verb)	hold on (verb)
look forward to the probable occurrence of	be persistent, refuse to stop
serve as a waiter or waitress in a restaurant	stop and wait, as if awaiting further instructions or developments
stay in one place and anticipate or expect something	hold the phone line open
wait before acting	hold firmly
	retain possession of