The following comments on haiku benefit notably from the insights of Kai Falkman (The String Untouched, translation of En Orörd Sträng, Ordfront, 2005).

Haiku is essentially a very short poem depicting a specific experience in nature or in a human context. It is contrasted with a related form, senryū, which tends to be about human foibles while haiku tend to be about nature -- senryū are often cynical or darkly humorous while haiku are serious.

The traditional Japanese rules for haiku require the use of 17 syllables grouped into three lines composed of respectively 5-7-5 syllables. These rules are applied in a multitude of languages by a worldwide "haiku movement" (cf World Haiku Club; Haiku International Association) [more]. The emphasis is clearly placed on succinctness and appropriateness, requiring extremely careful consideration of the pattern of words used and the effect they together create. The superfluous is excluded. In the words of Antoine de Saint Exupery, "Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away."

Non-Japanese haiku poets have explored an even more abbreviated 3-5-3 form of haiku, as explained by Keiko Imaoka (Forms in English Haiku) in discussing the linguistic circumstances that necessitate shorter English haiku to be more loosely structured than Japanese haiku:

Over the years, however, most haiku poets in North America have become aware that 17 English syllables convey a great deal more information than 17 Japanese syllables, and have come to write haiku in fewer syllables, most often in three segments that follow a short-long-short pattern without a rigid structure. This style is called by some "free-form" haiku.

The core feature of haiku is an experience described in a concrete image designed to evoke the same experience in the reader. A good haiku is not simply a static description. Three valued attributes are:

* embodiment of a transformation -- possibly with a surprising ending and/or a lingering poetic atmosphere. This may be catalyzed by describing an impression characteristic of one sensory organ through words normally descriptive of the impression through another. Images may be connected in a surprising way, possibly by changing perspectives calling for movement between them. Thinking is surprised and changes direction. Metaphors are however rarely used in haiku, because the image is expected to speak for itself and not be compared with something else in order to be accentuated or transformed in significance. However it is consequently recognized as a form that is wonderful for metaphorical descriptions

* capacity to hold several layers of meaning that may be discovered or explored -- possibly subsequently on reflection, or over a period of time. This may be achieved by using a proximate image like a fractal to imply the larger context of which it is a detail. Indirect insight is typical of haiku.

* act as a container for deep meaning, as characterized by a sense of poignancy, being touched, existential tragedy, or inevitability beyond conventional frameworks. It offers a value-charged integrative perspective.

Stress is placed on the concreteness of the images. Purely abstract or intellectual concepts are not considered valid haiku -- irrespective of their conformity with the formal rules or the value of the experience they may engender. Meaningful insights overtly expressed are considered as an imposition, potentially alienating to the reader. This is an implicit aesthetic that is discovered by a receptive sensitivity rather than an invasive technique. A degree of detachment or distance is valued. Although the concrete images may be anchored in the immediate or distant past -- perhaps specifically associated with a season -- the effect sought is an experience in the present moment, the immediate here and now.