I am going to start with the most basic question and give you my definition. The great thing about the arts is that everyone is entitled to their own opinion, so here I go presenting mine.
These are rules and I feel I need to have a word on the subject of rules. The more experienced writers will have a “pfft!” moment to the mention of the subject and say to yourselves “I don’t need rules—rules are limiting!” To you guys I say this: Perhaps you don’t but rules in the arts get a bad rap. They are not fences, rules are training wheels. They help you to get where you want to go til you can get there by yourself, instinctively. Rules really help learners and once they have internalised them, they can be discarded and the real play and creativity can begin, so chill.
I believe that haiku are primarily a means to transfer a revelatory insight on nature or a singular sensory experience from one mind to another. This is the aim of haiku in its finest moments. Good haiku show us something about the world which we did not know or had not intuited. This is asking a lot of seventeen or fewer syllables but that is the challenge.
This might be achieved in many ways but for me a solid haiku consists of two images—sometimes one, never three or more—that compare, contrast or associate (Reichhold, 2002, pg54). There are other styles too, like The Technique of the Improbable World, which presents the world in such an unusual way that we see something we have never seen before. The example Reichhold uses in her book Writing and Enjoying Haiku: A Hands-on Guide, is as follows:
colors of the day
blown away (Reichhold, 2002, pg68).
This one haiku brilliantly captures the mystery that is haiku.
Another important feature is the cut. It works a bit like tripping over a rock and landing on a $50 note. It is a gap that causes you to leap to the conclusion that is the meaning in each haiku. The cut should occur between the two parts and should sound like a clear break between one image and another; there should be juxtaposition. It is this that causes the reader’s mind to attempt to find a relationship between the parts and it is where the magic lies. This is also why three images don’t work. Haiku is a pas de deux not a ménage a trois.
A haiku should be no more than seventeen syllables, lest it become too wordy. About 12-13 seems to render the best effect in English and is probably the closest to the original Japanese works in brevity. Less is more in this arena and the minimalist haiku are often the most effective. Long-short-long works well for line length but it is by no means an absolute. A sense of openness is vital. If you are telling a whole story, then there is too much and you need to cut something. The reader is not stupid; they will follow your hints and thank you for your subtlety. Things should be implied but not spoken in a haiku; implication is everything in this form. The imagery should be concrete. Feelings, sensation, should be derived from image not told by the writer. If there are not two concrete images in you haiku, you need to go back and fix it. As long as the above categories are fulfilled, the number of lines is up to the poet. Remember though, breaks should serve the poem and not be made to just to make your haiku three lines. It might be worth looking into the poetic concept of enjambment for further ideas on this theme. The haiku should sound lovely but, please no rhyme. Rhyme is designed to aurally close off a set; this is the exact opposite of what you are trying to achieve in haiku and rhyme tends to take over in such small poems. Beautiful sounds and rhythm can be created using subtle application of assonance and consonance. Remember though, as in all art, if the reader can see your working, then you are going too hard.
Beyond this, everything is fair game. I do not believe that any subject should not be tackled and I believe urban subjects are just as worthy as natural ones. Now get writing!
Reichhold, J, 2002, Writing and Enjoying Haiku: A Hands-on Guide, Kodanshi International, Tokyo.