unrhymed Japanese poetic form consisting of 17 syllables arranged in three lines of 5, 7, and 5 syllables respectively. The term haiku is derived from the first element of the word haikai (a humorous form of renga, or linked-verse poem) and the second element of the word hokku (the initial stanza of a renga). The hokku, which set the tone of a renga, had to mention in its three lines such subjects as the season, time of day, and the dominant features of the landscape, making it almost an independent poem. The hokku (often interchangeably called haikai) became known as the haiku late in the 19th century, when it was entirely divested of its original function of opening a sequence of verse; today even the earlier hokku are usually called haiku.

Originally, the haiku form was restricted in subject matter to an objective description of nature suggestive of one of the seasons, evoking a definite, though unstated, emotional response. The form gained distinction in the 17th century, during the Tokugawa period, when the great master Bashō elevated the hokku, as it was then known, to a highly refined and conscious art. Haiku has since remained the most popular form in Japanese poetry. Later its subject range was broadened, but it remained an art of expressing much and suggesting more in the fewest possible words. Other outstanding haiku masters were Buson in the 18th century, Issa in the late 18th and early 19th centuries, Masaoka Shiki in the later 19th century, and Takahama Kyoshi and Kawahigashi Hekigotō in the late 19th and early 20th centuries. At the turn of the 21st century there were said to be a million Japanese who composed haiku under the guidance of a teacher.

A poem written in the haiku form or a modification of it in a language other than Japanese is also called a haiku. In English, the haiku composed by the Imagists (Imagist) were especially influential during the early 20th century. The form's popularity beyond Japan expanded significantly after World War II, and today haiku are written in a wide range of languages.