The kigo, or "season word", is an essential element in traditional Japanese haiku. Since haiku is, at its core, nature poetry, a haiku would contain a word to indicate the seasonal setting of the poem. Often, the kigo added to the subtlety of the poem by freeing the poet from providing a description of the scene. The season word came into use back when haiku was still hokku, the opening verse of renga. It was deemed necessary that the hokku indicate the date the renga was composed, and this gradually evolved into more subtle indicators of time.

The inclusion of a kigo is not as restrictive as it might sound. There are thousands of kigo to chose from, ranging from the obvious (like, say, "snow") to something associated through tradition with a particular season to things arbitrarily assigned to one. Poets could consult books of kigo: simple lists of words called kiyose or saijiki, lists accompanied with examples of usage. Today, you can consult web-based lists, like the impressive one at

Kigo fall into the following categories:

jiko ("season" or "climate") seasonal descriptions and effects, climate, temperature, time, etc.
tenmon ("astronomy") everything above the earth, the sky, stars, rain, wind, etc.
chiri ("geography") land and water
gyoji ("observances") religious and cultural holidays
seikatsu ("livelihood") work, fun, and rest
dobutsu ("animals") animals and insects
shokubutsu ("plants") flowers, trees, etc.

Sometimes the categories gyoji and seikatsu are combined into the category jinji, which is something like "human affairs".

The kigo is not so foreign a concept as it might sound. In Western poetry, a seemingly simple object, such as a nightingale or a willow, carries the weight of hundreds of years of allusions, and skilled poets are aware of this and chose their words accordingly.

The great master Matsuo Basho would do away the kigo in the occasional poem, but even so it truly is a central element of haiku. The radical innovator Masaoka Shiki, who eliminated most of the restrictive rules of haiku, retained the use of the kigo. Even his more radical follower Kawahigashi Hekigodo, who abandoned the 17-syllable count to write "free verse" haiku, continued to use the season word. Many later 20th century haiku poets, including some of Hekigodo’s followers, would do away with the kigo, however.

Writers of haiku in English and other Western languages have generally ignored the kigo.