seasonal word (in haiku)
俳句には必ず季語を入れる。
Haiku must always include a seasonal word.
花火は夏の季語としてよく使われる。
Fireworks are often used as a seasonal word for summer.
俳句を作るため、彼は季語の辞典で秋の季語を探した。
To compose a haiku, he looked up autumn seasonal words in a kigo dictionary.