hendecasyllabic
adjective
hen·deca·syl·lab·ic (ˌ)hen-ˌde-kə-sə-ˈla-bik 
: consisting of 11 syllables or composed of verses of 11 syllables
hendecasyllabic noun
hendecasyllable
hen-ˈde-kə-ˌsi-lə-bəl
noun (ˌ)hen-ˌde-kə-ˈsi-
Word History
Etymology
Latin hendecasyllabus, from Greek hendeka eleven (from hen-, heis one + deka ten) + syllabē syllable — more at same, ten
First Known Use
1704, in the meaning defined above