Decasyllable is a term used in syllabic verse . It means a line of ten (10) syllables.
Iambic pentameter, the verse most commonly used in English, is (normally) decasyllabic.
Line lengths in syllabic verse[]
8 - Octosyllable
10 - Decasyllable
11 - Hendecasyllable
12 - Dodecasyllable