IPA: //ˌɒk.təʊ.sɪˈlæb.ɪk//
KK: /ɑkˌtoʊsɪˈlæbɪk/
Describing a word or line of poetry that has eight syllables.
The poem was written in an octosyllabic form, making it rhythmically pleasing.
Octosyllabic → It is formed from "octo-" (from Greek "okto", meaning eight) and "syllabic" (from Latin "syllaba", meaning syllable). The word describes a word or line of verse that has eight syllables.
Think of 'eight' ('octo-') syllables in a line — that's what octosyllabic means.