A permissive license is an open source license that grants broad freedom to use, modify, and redistribute software while imposing only minimal conditions, most commonly the preservation of the copyright and license notices. The defining feature of a permissive license is what it does not require: it places no obligation on anyone who builds upon the code to release their own modifications under the same terms. This makes it possible to take permissively licensed code and incorporate it into a proprietary, closed-source product.
The clearest examples are the MIT License, the BSD family, the ISC License, and the Apache License 2.0. The MIT License illustrates the model directly: as published by the Open Source Initiative, it grants the rights to “use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software” subject only to the condition that “the above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.” Attribution is the entire price of admission.
Permissive licensing is the philosophical counterpoint to copyleft. A copyleft license, embodied by the GNU GPL, uses copyright to require that modified and extended versions carry the same freedoms forward, keeping the code and its freedoms legally inseparable. A permissive license declines to make that demand: it maximizes freedom for the next developer, including the freedom to make a derivative non-free. Advocates of copyleft argue this lets the public benefit of the code be captured privately; advocates of permissive licensing argue that the lower friction encourages the widest possible adoption, including by commercial vendors who would not touch copyleft code.
The Software Package Data Exchange (SPDX) maintains the canonical machine-readable catalog of these licenses, assigning each a stable short identifier such as MIT, BSD-3-Clause, ISC, or Apache-2.0, and marking whether each is approved by the Open Source Initiative or considered free by the Free Software Foundation. License-classification tools and tooling that scans dependency trees rely on these identifiers to tell, at scale, which components in a codebase are permissive and which carry copyleft obligations.
In practice the permissive category dominates modern package ecosystems. The brevity of these licenses, the absence of reciprocal obligations, and their compatibility with one another and with proprietary code have made them the default choice for libraries and frameworks, which is why permissive identifiers vastly outnumber copyleft ones across public code registries and source hosts.