Definition
A U.S. legal doctrine that allows limited use of copyrighted material without permission in some cases, such as commentary, criticism or teaching.
What it means
Fair use permits unlicensed use of copyright-protected works in certain circumstances, such as criticism, comment, news reporting, teaching and research. Courts decide it case by case using four factors: the purpose and character of the use, including whether it is commercial or transformative; the nature of the copyrighted work; the amount and substantiality of the portion used; and the effect of the use on the market for the original. There is no fixed number of seconds or words that is automatically fair. Giving credit does not by itself make a use fair. Creators who use clips, music or images from others should rely on licenses or permission unless they are confident the use is fair, because a mistaken claim can lead to takedowns or lawsuits.
Example
A reviewer who shows a few short clips of a movie while criticizing it may be making fair use; uploading the whole movie with a short comment would not be.
Illustrative numbers only. Your situation will differ.