What is Digital Millennium Copyright Act?

Legal Definition
The Digital Millennium Copyright Act (DMCA) is a United States copyright law that implements two 1996 treaties of the World Intellectual Property Organization (WIPO). It criminalizes production and dissemination of technology, devices, or services intended to circumvent measures (commonly known as digital rights management or DRM) that control access to copyrighted works. It also criminalizes the act of circumventing an access control, whether or not there is actual infringement of copyright itself. In addition, the DMCA heightens the penalties for copyright infringement on the Internet. Passed on October 12, 1998, by a unanimous vote in the United States Senate and signed into law by President Bill Clinton on October 28, 1998, the DMCA amended Title 17 of the United States Code to extend the reach of copyright, while limiting the liability of the providers of online services for copyright infringement by their users.

The DMCA's principal innovation in the field of copyright is the exemption from direct and indirect liability of Internet service providers and other intermediaries. This exemption was adopted by the European Union in the Electronic Commerce Directive 2000. The Copyright Directive 2001 implemented the 1996 WIPO Copyright Treaty in the EU.
-- Wikipedia
Legal Definition
The Digital Millennium Copyright Act (DMCA), Public Law 105-304, was signed into law on October 29, 1998, by President Clinton. It combined four proposed bills as well as some other revisions to copyright law.

The “WIPO Copyright and Performances and Phonograms Treaties Implementation Act of 1998” added Chapter 12, Copyright Protection and Management Systems, to the Copyright Act. While United States copyright law arguably met most of the provisions in the WIPO treatises, it did not address technological measures to help stop copyright infringement or other copyright management systems. The DMCA resolved that by first making it a violation to circumvent a technological measure to access a copyrighted work, and then making it a violation to traffic in devices whose purpose is to circumvent the technological measure to either access the work or otherwise infringe the copyright. There are a number of narrowly-tailored exceptions provided, as well as a triennial rulemaking by the Library of Congress to exempt classes of copyrighted works.

The “Online Copyright Infringement Liability Limitation Act” added Section 512 to the Copyright Act, providing four distinct safe harbors for online service providers. If the conduct of a service provider meets all the specific requirements of a safe harbor, the service provider will not be liable for monetary damages for copyright infringement, although injunctions to stop future infringement are possible. The safe harbor for user information residing on the service providers’ systems includes a “notice-takedown-putback” provision where the service provide takes down information after proper notice is received from the copyright owner alleging infringement, and can be restored if the user challenges the takedown after sufficient time to file a copyright infringement suit.

When most people refer to the DMCA, they are talking about one or both of those provisions. But it included many other things, one not even about digital information or copyright. The “Vessel Hull Design Protection Act” added Chapter 13 to Title 17. Although not a copyright provision, it gave the Copyright Office a new role is protecting an “original design of a useful article” (where “useful article” is limited to boat hulls). It was intended to replace the state law struck down by the Supreme Court in Bonito Boats, Inc. v. Thunder Craft Boats, Inc.

With the “Computer Maintenance Competition Assurance Act,” the DMCA also broadened Section 117 of the Copyright Act to allow third-party maintenance organizations to used software licensed to a computer’s owner or lessee as part of their maintenance activities, overriding the Ninth Circuit’s 1993 decision in Section 108 exemptions for libraries and archives.