A haiku is a short form of Japanese poetry in three phrases, typically following three rules:

• The juxtaposition of two images or ideas and a kireji ("cutting word") between them. The kireji signals the moment of separation and colours the manner in which the juxtaposed elements are related.
• Traditional haiku often consist of 17 syllables, in three groups of 5, 7, and 5, respectively.
• A seasonal reference.

However, 'good' haiku are said to contain other qualities:

• Ambiguity between subject and object.
• Candid, genuine emotion, not showy or 'clever'.
• A delicate, ego-less approach.

Another form, Senryū, generally addresses aspects of human nature in a satirical or comical manner.

The 5-7-5 structure should not be observed as a strict rule, lest it become too cramped a cage that compromises the poem's flow. It's the pursuit of essence, efficiency and the impression of spontaneity that make up the DNA of a haiku or senryū poem, not its syllable count.