The zlib license is a permissive free software license which defines the terms under which the zlib software library can be distributed. It is also used by many other free software packages. The libpng library uses a similar license sometimes referred interchangeably as the zlib/libpng license.The zlib license has been approved by the Free Software Foundation (FSF) as a free software licence, and by the Open Source Initiative (OSI) as an open source license.