I've also heard the syllables be referred to as
Onji : In traditional Japanese, the haiku was often written in one long string, from top to bottom, using 17
onji (sound-symbols), the shortest unit of meaning. These onji were usually divided into 3 sections, with the middle one being slightly longer than the others; a pause at the end of the first or second section often divided the haiku into two thoughts or images which contrasted or combined to make a striking perception, usually involving nature. Some call this the
Haiku Moment.
More tidbits from the web: Early translators, assuming that onji equaled syllables in our language
(they do not), decided the English equivalent should be 3 lines containing 5-7-5 syllables respectively. Many poets still look for haiku in that pattern, though occasionally today’s verses may have from 1 to 5 lines and a varying number of syllables. Not all classical Japanese haiku had 17 onji, let alone 17 “syllables,” so if we insist on that pattern, it is out of habit, not out of respect for the original.

~L