1. fill in the blank - Noun
2. fill in the blank - Verb
A language exercise in which one or more words are replaced with a blank line and the reader is tasked with supplying a valid replacement.
The test consisted of fill-in-the-blank sentences.
(idiomatic, figuratively) A placeholder allowing for any of a variety of possible answers or a single elicited answer.
To complete a single fill in the blank question.
(idiomatic, figuratively) To complete an interruption, ambiguity, or vagueness in understanding, perception, or context.
Source: en.wiktionary.org