What is the term for the 'cutting word' in haiku?

The term for the “cutting word” in haiku is kireji.

A kireji is a special Japanese word or particle that creates a pause, break, or emotional emphasis within a traditional haiku. It commonly appears at the end of one of the poem’s metrical phrases, where it can divide two images while also suggesting a relationship between them. At the end of a poem, it may provide a sense of closure or heightened feeling.

Kireji developed from conventions in renga and related forms of Japanese linked verse. Classical Japanese has a set of commonly recognized cutting words, including ya and kana, whose meanings depend partly on their position and grammatical role. Their effects are difficult to reproduce exactly in English.

English-language haiku often represents a kireji with a dash, ellipsis, line break, or punctuation—or leaves the pause implicit. Kireji is therefore not the same as kigo, which signals a season. Nor is it simply any punctuation mark: its traditional function combines rhythm, grammar, emphasis, and poetic juxtaposition.