Many people today define haiku as a three-lined poem comprised of 17 syllables. It is true that traditional Japanese haiku followed the form of 5-7-5 sound units, but in Japan, sound units (called kana) are shorter and counted differently than English syllables.
For example, in English, the word London is counted as two syllables, but in Japanese, the word would be broken down in the following way: lo/n/do/n. (See Stalking the Wild Onji, by Richard Gilbert, PH.D.)
This means that if an English language haiku comprised of 17 syllables was translated into Japanese, it would be counted as having more than 17 Japanese sound units. This is why English language haiku that follow the 5-7-5 form can sometimes end up feeling wordy.
Another issue that can affect the syllable count is the use of cutting words (called kireji in Japanese), which is a category of words that indicate pauses or emphasis at the end of a line.
Kireji are written/spoken punctuation and were included in the sound unit count. This could ultimately take up to two or even three sound units. Since punctuation in English language haiku is not included in the syllable count, this also causes another sound unit/syllable imbalance.
Most modern haiku experts today agree that approximately 11 - 14 syllables is a closer equivalency to the 17 sound units found in traditional Japanese haiku.
But instead of counting "syllables" when writing haiku, many believe that a good rule of thumb is to try to keep to the form of a short line, then long, and then short line.
Another option for teachers who would like to use haiku in relation to teaching syllables would be to go with a 3-5-3 syllable form.
When writing haiku, one should also remember to focus on the simple beauty of everyday moments. By using specific and descriptive vocabulary, poets can create vivid imagery that will come alive for the reader.
In the end, this is what the true essence of haiku is all about!