You can say e.g.:
The word "on" is a monosyllable.
but it seems that the word "multisyllable" has been outdated since 1913.
What is the correct term for a word that has two or more syllables, e.g.
"The word "beautiful" is a _________.
poly is usually opposite to mono: polysyllable
beautiful is a polysyllabic word. (not sure about this usage)
I believe you would say it is a multisyllabic word.
According to WordNet, "polysyllable" means a word having more than three syllables, so it is not correct for the general case.
"Polysyllabic" is an adjective used, e.g. for "polysyllabic humour", where, for example, instead of saying "coca cola" one says "an ebony coloured, effervescent beverage of the drinkable kind, flavoured with the frutiferous bounties of the cola plant" or something. This kind of "humour" was used by O. Henry etc. But for common usage, I have not heard the word "polysyllable" on its own and I would use "polysyllabic word" or "word with more than one syllable".
I don't know clearly how goes in english, but the origin is commonly latin/greek so you should stick with it:
1 Syllable - Mono (one-unique) Syllable
2 Syllable - Di (Bi-Duo-two) Syllable
3 Syllable - Tri (tres-three) Syllable
4 Syllable - Tetra (cuatro-four) Syllable
5 Syllable - Penta (five) Syllable
More than one - Poly (many) Syllable
As in Polygamy (many couples) As in Polygon (many sides)