Definition -
(1) (adjective) A type of constrained writing where the writer only uses words that contain one particular vowel, e.g. the writer only uses words that contain the vowel a.
(2) (noun) A piece of univocalic writing.
Note: As opposed to a lipogram in which the writer avoids using any words that contain a certain letter or letters.

Example -

No cool monsoons blow soft on Oxford dons,
Orthodox, jog-trot, book-worm Solomons
(The above are lines from 1890 poem "Incontrovertible Facts" by C.C. Bombaugh; it is a univocalic constrained to the vowel O.)

Etymology -
The term derives from the Latin uni, one + vocalic, that which relates to vowels (from vox, voice). It was probably coined by C.C. Bombaugh who was an avid practitioner (see the OED citation below).

Oxford English Dictionary -
Its first citation is from 1867:
"Univocalic verses…. Wars harm all ranks, all arts, all crafts appall."
(C. C. Bombaugh Gleanings (ed. 3), 20)

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