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.
Constraints on writing are common and can serve a variety of purposes. For example, a text may place restrictions on its vocabulary, e.g. Basic English, copula-free text, defining vocabulary for dictionaries, and other limited vocabularies for teaching English as a second language or to children.
In poetry, formal constraints abound in both mainstream and experimental work. Familiar elements of poetry like rhyme and meter are often applied as constraints. Well-established verse forms like the sonnet, sestina, villanelle, limerick, and haiku are variously constrained by meter, rhyme, repetition, length, and other characteristics.
Wikipedia contributors. (2018, November 7). Constrained writing. In Wikipedia, The Free Encyclopedia. Retrieved 17:50, March 15, 2019, from https://en.wikipedia.org/w/index.php?title=Constrained_writing&oldid=867702082
Note The definition in the usage guidance is also from Wikipedia.
