Haikai (seventeen-syllable verse) (俳諧)

Haikai is a form of Japanese literature (or a work of haikai) that generally flourished during the Edo period. The word haikai ("俳諧") is also written with the characters "誹諧." Strictly speaking, it is called haikai no renga (humorous linked poem) or haikai renga, which diverged from the orthodox renga, being made more playful, so it's a form of group literature as well as a general term for the forms of hokku (the first line of a waka poem) and renku (a linked verse). It became common to appreciate the hokku only, which came to be the origin of haiku established during the Meiji period. Hokku, as an individual art of creation, can be identified as haiku, which is a form of modern literature that became completely independent from hokku.
Those who were engaged in creating haikai professionally were called 'haikai poets.'

In the Edo period, the professional poets, or so-called 'gyohai,' were referred to as haikai poets.

Those who enjoyed haikai as a hobby were called 'yuhai,' but a yuhai was not considered to be a haikai poet.

History
The word 'haikai' originally referred to 'humorous' or 'a play.'
The humorous poems collected in "Kokin Wakashu" (A Collection of Ancient and Modern Japanese Poetry) were called 'haikaika' (humorous poems).

The form of literature in which expressions in renga (linked verse) were made more humorous and sophisticated in order to enjoy them readily was called 'haikai renga' or 'haikai no renga,' and it flourished. It had been considered just a branch form of renga, but it prospered with the work of Sokan YAMAZAKI and other people.

Haikai renga was brought to perfection by Teitoku MATSUNAGA during the Edo period. Haikai renga, as created by the Teitoku School, were called 'Teimon ha' (Teimon School), characterizing a period in history, and their popularity nearly exceeded that of the stiff, orthodox renga. However, in opposition to the 'antique style' of poetry made by Teitoku, a 'new style' appeared and deprived the Teimon School of its high social status. The new style of poetry was called 'Danrin ha' (Danrin school), and Saikaku IHARA, who established ukiyo zoshi (popular stories of everyday life in the Edo period), joined the school with the renga poet Soin NISHIYAMA at the head of the list.
After the Danrin school finished its short golden age, Basho MATSUO appeared and established the new style called 'bafu.'
In contrast to 'kotoba zuke,' the style of the Teimon school, and 'kokoro zuke,' the style of the Danrin school, bafu was considered 'nioi zuke' (adding atmosphere).

After the death of Basho, zappai (playful literature originating from haiku) flourished temporarily, especially senryu (humorous poem) in which poets competed with each other to make a technically accomplished follow-up line. However, haikai regained its liveliness with the work of Buson YOSA, who restored the form.

At the end of the Edo period, Issa KOBAYASHI was actively involved in haikai.

Throughout the Edo period, haikai's main form was a linked-verse style, which was not changed even though people sometimes appreciated only hokku. In the Meiji period, however, Shiki MASAOKA brought the traditional literature, haikai renga, to the status of the modern haiku composed by a single poet, making hokku independent from renga.

After the establishment of haiku, when the traditional group literature of renga was composed as modern literature, it came to be called renku (a linked verse) in order to distinguish it from haiku.

Form
Haikai was originally composed as a part of renga, but occasionally it has been considered that only hokku became independent from renga. Later, the hokku for renku came to be called 'tateku,' while an independent hokku was called 'jihokku' in order to distinguish them.

The characteristics of haikai are that Haikai renga, or renku, is a work that should be treated as connected verses, and moreover, the composers and the audience shared the same room due to the fact that several people have to be involved in completing the verses continuously. Therefore, the original haikai as a part of renga was appreciated from a different perspective than haiku, which appeared after the modern times.