2 vs pair

2 and pair both are nouns.

Word	Noun	Adjective	Verb	Adverb
2	Yes	Yes	No	No
pair	Yes	No	Yes	No

As nouns, pair is a hyponym of 2; that is, pair is a word with a more specific, narrower meaning than 2:

2: the cardinal number that is the sum of one and one or a numeral representing this number
pair: two items of the same kind

Other hyponyms of 2 include craps, snake eyes, brace, couple, couplet, distich, duad, duet, duo, dyad, span, twain, twosome, yoke.

2 (noun)	pair (noun)
the cardinal number that is the sum of one and one or a numeral representing this number	two people considered as a unit
	a set of two similar things considered as a unit
	a poker hand with 2 cards of the same value
	two items of the same kind

2 (adjective)	pair (adjective)
being one more than one

2 (verb)	pair (verb)
	engage in sexual intercourse
	bring two objects, ideas, or people together
	arrange in pairs
	occur in pairs
	form a pair or pairs