yesterday vs past

yesterday and past both are nouns.

Word	Noun	Adjective	Verb	Adverb
yesterday	Yes	No	No	Yes
past	Yes	Yes	No	Yes

As nouns, past is a hypernym of yesterday; that is, past is a word with a broader meaning than yesterday:

Other hypernyms of yesterday include past times, yesteryear.

yesterday (noun)	past (noun)
the recent past	a verb tense that expresses actions or states in the past
the day immediately before today	the time that has elapsed
	a earlier period in someone's life (especially one that they have reason to keep secret)

yesterday (adjective)	past (adjective)
	earlier than the present time; no longer current
	of a person who has held and relinquished a position or office

yesterday (adverb)	past (adverb)
on the day preceding today	so as to pass a given point
in the recent past; only a short time ago