IPA: /ˌpɒlɪˈsɪbəl/
KK: /pɑːlɪˈsɪbəl/
A word that has more than one syllable, typically more than three.
The word 'unbelievable' is a polysyllable because it has five syllables.
Polysyllable → It is formed from "poly-" (from Greek "polus", meaning many) and "syllable" (from Latin "syllaba", meaning a unit of speech). The word "polysyllable" refers to a word that has many syllables.
Think of 'many' ('poly-') syllables in a word to remember that a polysyllable is a word with multiple syllables.