A word or phrase that indicates a particular season, used in traditional Japanese poetry such as haiku and renga. Each 季語 is associated with a specific season and is collected in reference books called 歳時記.
You always include one seasonal word in a haiku.
「雪」は冬の季語だ。
"Snow" is a winter seasonal word.
It's a basic rule of haiku to include a seasonal word.
This poem uses the seasonal word 'sakura' (cherry blossom).
He skillfully wove a seasonal word into his haiku.
This poem has no seasonal word, so it's called a 'kigo-less' haiku.