"Haiku" is a term, at times, [loosely] applied to any short, impressionistic poem, however there are certain characteristics that are commonly associated with the genre, such as having a strong focus on nature or the four seasons. The haiku often has a contemplative or wistful tone, and/or a vague briefness to the setting, that might inspire the poet. There may be a division involving two asymmetrical sections that juxtaposes two subjects, such as the natural world as opposed to the human made. In other words, two unexpectedly similar things are being compared one to the other.
