IPA: //ˌɒk.təˈsɪl.ə.bəl//
KK: /ˈɑktoʊˌsɪləbəl/
A word or line that has eight syllables.
The poem was written in octosyllables, giving it a rhythmic flow.
Octosyllable → It is formed from "octo-" (from Greek "okto", meaning eight) and "syllable" (from Latin "syllaba", meaning a unit of sound in a word). The word "octosyllable" refers to a line of verse that contains eight syllables.
Think of the prefix 'octo-' meaning eight, and remember that an 'octosyllable' is a line of poetry with eight sound units.