A gap-fill is a practice exercise in which learners have to replace words missing from a text. These words are chosen and removed in order to practise a specific language point. Gap-fill exercises contrast with cloze texts, where words are removed at regular intervals, e.g. every five words.

Example
The first sentence above prepared to practise article use could read '______ gap-fill is ______ practice exercise where ______learners have to replace ______ words which have been removed from ______ text.'

In the classroom
Gap-fills are often used to practise specific language points, for example items of grammar and vocabulary, and features of written texts such as conjunctions. They are common in testing.

Further links:

https://www.teachingenglish.org.uk/article/gap-fill-gamble

https://www.teachingenglish.org.uk/article/gap-fill-poems