a DMCA violation is any violation of the Digital Millennium Copyright Act. Its most typically refers to an online copyright violation by a third party, which the law provides procedures and actions to remedy.
When a copyright holder learns of a violation, a DMCA takedown notice is issued to the service that hosts the offending website or to the internet service provider (ISP) of the violator. Infringing material can also be removed from search results by issuing a notice to a search engine.