The general definition of an English haiku is a 3 line poem in a 5-7-5 syllable structure. However this is actually a bit too simple statement.
Here is a walkthrough, mainly referenced from
Ray Rasmussen's definition.
BrevityAn important component of a haiku's beauty is that it is minimalistic, short but still meaningful. A haiku is limited to use one to three lines, totaling 17 syllables or less.
When read aloud, a haiku can be completed in one breath.
SymmetryAs stated above, the general requirement is three lines made of 5-7-5 syllables. However, some people actually suggest a structure of three lines with 5 syllables or less, 7 or less and 5 syllables or less.
ContentA haiky should avoid traditional English poetic forms, such as rhyming and metephor. In addition, a haiku describes it does not prescribe or tell.
Finally, a vital part in the signature of a haiku is that two elements or lines of the haiku relates to a third.
Coffee HaikuSo let us consider the Coffee Haiku from my earlier post.. Does it meet all requirements as far as Brevity, Symmetry and Content? Well almost, there is actually a syllable too much in the third line. However, who really cares. In this poem, the awaiting coffee is what matters.