Haiku is a Japanese poetic form whose origins trace to the collaborative linked-verse poetry tradition called renga, which flourished in Japan from the 13th century onward. In renga, poets would take turns composing alternating verses of 5-7-5 and 7-7 syllables (more precisely, "on" or morae in Japanese). The opening verse of a renga, called the hokku, held special significance: it set the tone, referenced the season, and stood as a self-contained poetic statement. Over centuries, the hokku evolved into an independent form, eventually named "haiku" by the poet and critic Masaoka Shiki in the late 19th century.

Matsuo Basho (1644-1694) is widely considered the greatest haiku master. His famous frog haiku — "Furu ike ya / kawazu tobikomu / mizu no oto" (The old pond / a frog jumps in / the sound of water) — exemplifies the form's essential qualities: a concrete natural image, a moment of sensory perception, and an implied depth of meaning beyond the literal words. Basho elevated haiku from a literary game to a serious art form by infusing it with Zen Buddhist aesthetics, particularly the concepts of wabi (rustic simplicity) and sabi (the beauty of impermanence and aging).

Two structural elements define traditional haiku beyond the syllable count. Kigo (seasonal words) are references that place the poem in a specific season — "cherry blossoms" indicates spring, "cicada" indicates summer, "harvest moon" indicates autumn, and "snowfall" indicates winter. Japanese haiku almanacs (saijiki) catalog thousands of kigo with their seasonal associations. Kireji (cutting words) are linguistic particles that create a pause or juxtaposition within the poem, dividing it into two contrasting or complementary images. In English haiku, kireji are often represented by a dash, ellipsis, or line break rather than a specific word.

The 5-7-5 syllable rule requires nuance when applied to English. Japanese "on" (morae) are not identical to English syllables — they are units of sound timing, where each mora takes roughly equal time to speak. The Japanese word "Osaka" has four morae (o-sa-ka) but three English syllables. Because English syllables carry more information than Japanese morae, many English-language haiku poets argue that a 5-7-5 English haiku is actually longer than its Japanese equivalent and advocate for shorter forms (often around 10-14 syllables total). The Haiku Society of America's definition does not specify a syllable count, instead emphasizing brevity and a two-part structure with juxtaposition.

Other notable haiku masters include Yosa Buson (1716-1784), known for his painterly visual imagery; Kobayashi Issa (1763-1828), celebrated for his compassionate, often humorous poems about small creatures and everyday life; and Masaoka Shiki (1867-1902), who modernized haiku by advocating for direct observation (shasei, "sketching from life") and rejecting the mysticism that had come to dominate the form. In the 20th century, haiku spread globally, with significant English-language traditions developing in North America and Europe, and contemporary poets continue to explore how this ancient form can capture modern experience.