Fair use
A rule of US copyright law that allows some use of copyrighted material without the owner's permission. Only a court decides it, case by case.
Fair use comes from US law. Other countries have exceptions of their own, often similar but not the same. Courts weigh four factors: why and how the material is used (commentary and new meaning count for it, plain copying against it, and education more than commerce), what kind of work it is (factual more than fiction), how much is taken, and whether the use hurts the owner's ability to earn from the original.
Nothing automatic can decide it: YouTube says Content ID cannot judge fair use, because only a court can. Giving credit does not by itself turn a copy into fair use, and writing "no infringement intended" does not protect a video. And fair use is a question about copyright only: YouTube's reused content rule for monetization applies whatever the answer.
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