peak vs upper limit

peak and upper limit both are nouns.

Word	Noun	Adjective	Verb	Adverb
peak	Yes	No	Yes	No
upper limit	Yes	No	No	No

As nouns, upper limit is a hyponym of peak; that is, upper limit is a word with a more specific, narrower meaning than peak:

Other hyponyms of peak include lower limit, minimum, maximum.

peak (noun)	upper limit (noun)
a brim that projects to the front to shade the eyes	the limit on the upper (or northernmost) side of something
the top or extreme point of something (usually a mountain or hill)	the largest possible quantity
the highest point (of something)
the most extreme possible amount or value
a V shape
the highest level or degree attainable; the highest stage of development
the period of greatest prosperity or productivity

peak (verb)	upper limit (verb)
to reach the highest point; attain maximum intensity, activity