What is Patent Cooperation Treaty?

Legal Definition
The Patent Cooperation Treaty (PCT) is an international patent law treaty, concluded in 1970. It provides a unified procedure for filing patent applications to protect inventions in each of its contracting states. A patent application filed under the PCT is called an international application, or PCT application.

A single filing of a PCT application is made with a Receiving Office (RO) in one language. It then results in a search performed by an International Searching Authority (ISA), accompanied by a written opinion regarding the patentability of the invention, which is the subject of the application. It is optionally followed by a preliminary examination, performed by an International Preliminary Examining Authority (IPEA). Finally, the relevant national or regional authorities administer matters related to the examination of application (if provided by national law) and issuance of patent.

A PCT application does not itself result in the grant of a patent, since there is no such thing as an "international patent", and the grant of patent is a prerogative of each national or regional authority. In other words, a PCT application, which establishes a filing date in all contracting states, must be followed up with the step of entering into national or regional phases to proceed towards grant of one or more patents. The PCT procedure essentially leads to a standard national or regional patent application, which may be granted or rejected according to applicable law, in each jurisdiction in which a patent is desired.

The contracting states, the states which are parties to the PCT, constitute the International Patent Cooperation Union.
-- Wikipedia
Legal Definition
Extended to all those who signed at the Paris Convention, this treaty was published in 1978, and provides the requirements of procedure for obtaining a patent, making it easier for inventors to file applications.