A kigo is a word or phrase used in Japanese poetry that evokes a particular season. They are one of the three defining qualities of the classic Japanese Haiku, along with a sense of juxtaposition and the 5-7-5 structure that is all most Anglophones know about them. Shallow Parody of haiku (or Japanese poetry as a whole) overwhelmingly uses cherry blossoms in this role, which bloom for a fleeting window of time in early spring. Mentioning cherry blossoms is an economical way of setting the season, and haiku is all about economy of expression.
Other common kigo are:
Spring: Warm (atatakashi or nurumu), the warming weather; uguisu, Japanese bush warbler, regarded as a harbinger of spring; Frogs: especially their calling
Summer: Hot/heat; Rainy season starting in mid-June; Wisteria which blooms in early summer and lotus in late summer.
Autumn (Fall): Coloured leaves; Typhoon, typhoon season is roughly June-December; Insects, implying singing insects such as crickets.
Winter: Snow, Cold, fugu soup, a seasonal dish, Christmas (modern).
Not to be confused with Keigo.