Definition
The Apache License 2.0 is a permissive open-source license maintained by the Apache Software Foundation. Like other permissive licenses it allows broad reuse, modification, and redistribution in source or object form, including in proprietary products, while disclaiming warranty. Its defining features are an explicit patent grant and a structured attribution mechanism.
The patent grant
Each contributor grants a perpetual, worldwide, royalty-free, irrevocable patent license covering their contributions. Crucially, that grant terminates automatically for anyone who initiates patent litigation alleging the work infringes a patent. This built-in patent retaliation clause gives downstream users protection that the simpler MIT License does not provide, which is why Apache 2.0 is often chosen for larger, multi-contributor projects.
The NOTICE file
If a work ships with a NOTICE text file, redistributors of derivative works must reproduce its attribution notices, though they may add their own. The NOTICE contents are informational only and do not modify the license terms.
Apache 2.0 is not copyleft: derivative works may be closed or re-licensed, so long as the license text, any NOTICE, and a statement of changes accompany the distribution. It is GPLv3-compatible, allowing Apache-licensed code to be combined into GPLv3 projects. When recording it in metadata, its SPDX identifier is Apache-2.0.
In Simple Terms
The Apache License 2.0 is a permissive open-source license maintained by the Apache Software Foundation. Like other permissive licenses it allows broad reuse, modification, and…