II vs dyad

II and dyad both are nouns.

Word	Noun	Adjective	Verb	Adverb
II	Yes	No	No	No
dyad	Yes	No	No	No

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

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

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

II (noun)	dyad (noun)
the cardinal number that is the sum of one and one or a numeral representing this number	two items of the same kind