A univocalic is a type of constrained writing 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, 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:
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 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's text Eunoia, published by Coach House Press in 2001.
An example of a univocalic novella is Georges Perec's Les Revenentes (sic), in which the vowel "e" is used exclusively.