A polysyllable is the noun: it is a word with three or more beats. Elephant is a polysyllable. You is not. Teachers and dictionaries use this when they sort words by length in the ear, not by how many letters you see. Though actually, the counting is a little messy. Some writers call any word with more than one beat a polysyllable; school books often wait until the third beat. You almost never need this word in daily talk. If you want to say “a long word,” people will understand you. Use polysyllable when you are naming the thing itself, the way you would say “a compound” or “a verb.”