Haiku
The traditional hokku usually was written in six verses or more or less 5, 7, 5 syllables and they had to follow (on-ji).  The Japanese word cow, meaning "sound", corresponds to a mora, a phonetic unit similar but not identical to the syllable of a language such as English.  A haiku has a special season word (the kigo) to represent the season in which the poem is set, or a reference to the natural world.