Attribution: original WP article
A YouTube copyright strike is a copyright policing practice used by YouTube for the purpose of managing copyright infringement and complying with the Digital Millennium Copyright Act. The Digital Millennium Copyright Act (DMCA) is the basis for the design of the YouTube copyright strike system. For YouTube to retain DMCA safe harbor protection, it must respond to copyright infringement claims with a notice and take down process. YouTube's own practice is to issue a "YouTube copyright strike" on the user accused of copyright infringement. When a YouTube user has 3 strikes, then YouTube cancels all of that user's YouTube accounts, removes all of their videos, and refuses to allow that user to have another YouTube account.
Some users have expressed concern that the strike process is unfair to users. The complaint is that the system assumes guilt of YouTube users and takes the side of copyright holders even when no infringement has occurred.
YouTube and Nintendo were criticized by Cory Doctorow, a writer for the blog Boing Boing, due to them reportedly treating video game reviewers unfairly by threatening them with strikes.
Copyright strikes have also been issued against creators themselves, as happened when Miracle of Sound's channel was hit with multiple copyright strikes as a result of automated strikes by the distributor of their own music.
If a user submits a claim that YouTube deems dubious, YouTube support will send the claimant an email mentioning that they should either retract the claim or reply with follow-up as an oath that the claim is accurate all within one week to prevent termination. Ignoring it however will not result in termination as they seem to forget to do so.
Nonetheless, if a copyright claim is too good to be true, YouTube will generally err on the side of caution and take down the video. This can result in termination, with a message saying "This account has been terminated because we received multiple third-party claims of copyright infringement regarding material the user posted." (Japanese: 「このアカウントを停止しました。これは、このユーザーのコンテンツが著作権を侵害しているとの申し立てが第三者から複数寄せられたためです。」), as has happened in these two examples.