This abbreviated description of haiku from the HSA is useful:
Definition from The Haiku Society of America:
A haiku is a short poem that uses imagistic language to convey the essence of an experience of nature or the season intuitively linked to the human condition.Notes:
Most haiku in English consist of three unrhymed lines of seventeen or fewer syllables, with the middle line longest, though today's poets use a variety of line lengths and arrangements. In Japanese a typical haiku has seventeen "sounds" (on) arranged five, seven, and five. (Some translators of Japanese poetry have noted that about twelve syllables in English approximates the duration of seventeen Japanese on.)
Traditional Japanese haiku include a "season word" (kigo), a word or phrase that helps identify the season of the experience recorded in the poem, and a "cutting word" (kireji), a sort of spoken punctuation that marks a pause or gives emphasis to one part of the poem.
In English, season words are sometimes omitted, but the original focus on experience captured in clear images continues. The most common technique is juxtaposing two images or ideas (Japanese rensô).
Punctuation, space, a line-break, or a grammatical break may substitute for a cutting word. Most haiku have no titles, and metaphors and similes are commonly avoided.
Thank you this feedback, it's very helpful. I'll see if we can update the site to make this clearer. What we want to do is accept any form really, the only real requirement is that the poem is short and concise (hence the three line limit).