Hokku (Japanese: 発句; literally starting verse). The first 17-on (sound) verse of a renga/renku or more generally, the first stanza of a Japanese-style linked poem. The term is sometimes used in the West to denote haiku, especially haiku that are composed in accordance with Japanese conventions.
1. The opening stanza of a renga/renku (連歌 / 連句, collaborative linked 俳諧 haikai sequence), nearly always in three phrases of five, seven, and five on (音 sounds; earlier called 音字 onji, phonetic symbols; properly morae but commonly called syllables in English]) and containing a kigo (季語 seasonal reference). The hokku was originally composed ex tempore at a linked-verse-writing occasion by a visiting renga master and was expected to set the season and mood for the renga, as well as include a compliment to the host of the event. Gradually the renga masters found it expeditious to compose starting verses in anticipation of an appropriate occasion, thus hokku evolved into the modern haiku.
2. In some Western usage, hokku refers to those poems written in the haikai manner by Japanese poets prior to the 20th century. Hence, Matsuo Bashō wrote not haiku, but hokku.
Note: This brief definition represents an abstract of a full-length Haikupedia glossary article to come.