Haiku (俳句) is a mode of Japanese poetry, a late 19th century revision by Masaoka Shiki of the older hokku (発句), the opening verse of a linked verse form, haikai no renga . A traditional hokku consists of a pattern of approximately 5, 7, and 5 morae, phonetic units which only partially correspond to the syllables of languages such as English. It also contains a special season word (the kigo) descriptive of the season in which the renga is set.
Hokku often combine two (or rarely, three) different elements into a unified sensory impression, with a major grammatical break (kire) usually at the end of either the first five or second seven morae. These elements of the older hokku are considered by many to be essential to haiku as well, although not always included by modern writers of Japanese "free-form haiku" and of non-Japanese haiku. Senryu is a similar poetry form that emphasizes humor and human foibles instead of seasons.