On March 21, 2007, Dr. Richard Gilbert of Kumamoto University gave a lecture about Japanese haiku. The following quotations and references are taken from page 4 of the handout from that lecture.
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"In 1999, Hiroaki Sato wrote: 'Today it may be possible to describe haiku but not define it. This is indicated by the haiku dictionary Gendai Haiku Dai-jiten ([Encyclopedic Dictionary of Contemporary Haiku]), Meiji Shoi, 1980). Its entry on haiku describes the history of the term, but makes no attempt to say what a haiku is. Both in form and content, all you can say is that a haiku, be it composed in Japanese, English or any other language, is what the person who has written it presents as a haiku.'"
"One of the early pioneers of haiku in English, Harold Henderson (1958) has written, 'A definitive definition of haiku is probably impossible [as haiku] must be what poets make them, not verses that follow 'rules' set down by some 'authority' . . . a strict definition is neither possible nor desirable" (46-47); later adding (correctly) that 'the rigid 17-syllable [and 5-7-5 on] requirement does not exist in Japanese" (1971).
Henderson, H.G. (1958). Introduction to Haiku. New York: Doubleday. (updated version of Bamboo Broom, 1934)
________. (1971). "Letter to Anita Virgil, dated May 5, 1971." In Haiku Society of America (1994). A Haiku Path. (p. 46). New York: Haiku Society of America, Inc.
Sato, Hiroaki. (1999). "HSA definition reconsidered." Frogpond: Journal of the Haiku Society of America (22:3), 73.