A license (American English) or licence (Commonwealth English) is a document or agreement giving permission to do something.
In Law, the Document is the evidence of a license to be distinguished from the underlying license which is the actual permission to an act in a way that would be otherwise unlawful. Originally in reference to property, a license was the right of an individual to enter upon the property of another to do an act that would have otherwise been considered illegal as a Trespass, such as walking in the woods, hunting game or swimming in the lake. To be distinguished from a license coupled with an interest which is an irrevocable license that granted some interest in land or in a chattel. Such a license could be enforced with an Injunction. Licenses can be gratuitous, revokable at will (sometimes called a bare license) or a type of Bailment.
The holder of a Copyright, Trademark, or Patent may (and often does) require that a license be accepted as a condition of being allowed to use, reproduce, or create an instance of the licensed work.
Computer users may think of licenses as in reference to end user licence agreements (EULA), which are claimed by vendors to encumber the user with extra restrictions besides the copyright, as a condition of granting permission under copyright law to use the work. The person who purchases a book normally owns the atoms and the right to resell or lend, but not the copyright to the text. In the United States this right to resell is part of the First-sale doctrine.
Software licenses are often highly restrictive, and most software users do not read them in full. So-called "shrink-wrap" licenses and "click-through" licenses are common. Most limit the number of computers the software can be installed on, the number of users that can use the software, and apply other limitations that are not inherent in the technology. As a result, huge fortunes have been made by selling goods that have a minimal cost of reproduction on a per-item basis.
In the US, the First-sale doctrine, Softman v. Adobe and Novell, Inc. v. CPU Distrib., Inc. rule that software sales are purchases, not licenses, and resale, including unbundling, is lawful regardless of a contractual prohibition.
So-called free software licenses and open source licenses are a reaction to what many see as the unfair restrictions of proprietary software licenses.
A license is an Academic degree in many European universities which is sort of equivalent to the Master's degree. Originally, in order to teach at a university, one needed this degree which, according to its title, gave you a license to teach. The name survived despite the fact that nowadays one really needs a Doctorate in order to teach at a university. A person who holds a license is called a Licentiate. Currently, licenciate degree is a sort of middle-level degree between a master's degree and a doctorate.