A univocalic is a type of constrained writing

Constrained writing

Constrained writing is a literary technique in which the writer is bound by some condition that forbids certain things or imposes a pattern.Constraints are very common in poetry, which often requires the writer to use a particular verse form....

 that uses only one vowel-letter (i.e., only uses one of A, E, I, O, or U). It can thus be considered a lipogram

Lipogram

A lipogram is a kind of constrained writing or word game consisting of writing paragraphs or longer works in which a particular letter or group of letters is avoided — usually a common vowel, and frequently "E", the most common letter in the English language.Writing a lipogram is a trivial task...

, excluding the other four vowels.

One of the best-known univocalic poems was written by C.C. Bombaugh in 1890, centred on the vowel "o". Here's a sample couplet:

No cool monsoons blow soft on Oxford dons,

Orthodox, jog-trot, book-worm Solomons

Bombaugh's work is still in print, and his Gleanings from the Harvest-Fields of Literature in particular is full of oddities of this kind.

The Austrian poet Ernst Jandl

Ernst Jandl

Ernst Jandl was an Austrian writer, poet, and translator.- Poetry :Influenced by Dada he started to write experimental poetry, first published in the journal "Neue Wege" in 1952....

 composed his univocalic poem "Ottos Mops" (Otto's Pug) from German words with only the vowel "o".

A contemporary example of English-language univocalic poems is Canadian poet Christian Bök

Christian Bök

Christian Bök is an experimental Canadian poet. He is the author of Eunoia, which won the Canadian Griffin Poetry Prize, and which has been said to be "Canada's best-selling poetry book ever."-Life:...

's text Eunoia

Eunoia (book)

Eunoia is the title of a set of univocalics by Canadian poet Christian Bök, which consists of chapters written using words limited to a single vowel...

, published by Coach House Press in 2001.

An example of a univocalic novella

Novella

A novella is a written, fictional, prose narrative usually longer than a novelette but shorter than a novel. The Science Fiction and Fantasy Writers of America Nebula Awards for science fiction define the novella as having a word count between 17,500 and 40,000...

 is Georges Perec

Georges Perec

Georges Perec was a French novelist, filmmaker, documentalist and essayist. He is a member of the Oulipo group...

's Les Revenentes (sic), in which the vowel "e" is used exclusively.