The term for a seasonal reference in haiku is kigo.
A kigo is a word or phrase associated with a particular season in Japanese poetry. It may name something obvious, such as snow for winter or cherry blossoms for spring, but it can also refer to cultural events, animals, plants, weather, or customs. The term literally means “season word.”
Kigo became especially important in haiku and in linked-verse forms such as renga and renku because a single seasonal image could evoke a large network of shared cultural associations. Japanese poets traditionally consulted a saijiki, an almanac listing season words along with explanations and example poems.
Kigo is sometimes confused with kireji, the “cutting word” that creates a pause or structural break in a haiku. They serve different purposes: kigo establishes seasonality, while kireji shapes the poem’s rhythm and meaning. Although many traditional haiku use kigo, modern Japanese and English-language haiku can deliberately omit it.