Kigo (季語, "season word"?) (plural kigo) is a word or phrase associated with a particular season, used in Japanese poetry. Kigo are used in the collaborative linked-verse forms renga and renku, as well as in haiku, to indicate the season referred to in the stanza. They are valuable in providing economy of expression.
Read more about Kigo: History of Kigo, Kigo and Seasons, Saijiki, Common Kigo in Japanese Haiku, Kigo Outside of Japan, Kigo and Haiku: An Example, Must Haiku Include A Kigo?