What is "dodecasyllable"

Dodecasyllable \Do*dec"a*syl`la*ble\, n. A word consisting of twelve syllables.

n. A word consisting of twelve syllables

Dodecasyllable verse is a line of verse with twelve syllables. 12 syllable lines are used in a variety of poetic traditions.

Jacob of Serugh (c. 451 – 29 November 521), a Miaphysite Bishop of Batnan da-Srugh, also called 'Flute of the Spirit' who composed in the dodecasyllabic verse more than seven-hundred verse homilies, or mêmrê (ܡܐܡܖ̈ܐ), of which only 225 have thus far been edited and published.

It is also used in Italian and French poetry, and in poetry of the Southern Slavs (the most famous example being Marko Marulić). In an Anglo-Saxon and French context, the dodecasyllable is generally called the " alexandrine", after the French equivalent.