- The essence of haiku is "cutting" (kiru). This is often represented by the juxtaposition of two images or ideas and a kireji ("cutting word") between them, a kind of verbal punctuation mark which signals the moment of separation and colours the manner in which the juxtaposed elements are related.
- Traditional haiku consist of 17 on (also known as morae), in three phrases of 5, 7 and 5 on respectively. Any one of the three phrases may end with the kireji. Although haiku are often stated to have 17 syllables, this is incorrect as syllables and on are not the same.
- A kigo (seasonal reference), usually drawn from a saijiki, an extensive but defined list of such words. The majority of kigo, but not all, are drawn from the natural world. This, combined with the origins of haiku in pre-industrial Japan, has led to the inaccurate impression that haiku are necessarily nature poems.
In Japanese, haiku are traditionally printed in a single vertical line while haiku in English often appear in three lines to parallel the three phrases of Japanese haiku.