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WordPress is Free Software; for more info about the very clear idea of the differences between Free Software and Open Source, please read:

For more info about this, please read:

WordPress is software licensed under the General Public License (GPL), which provides the user with four core freedoms:

  1. The freedom to run – WordPress can be run by anyone for any purpose
  2. The freedom to study – Anyone can study any aspect of the WordPress code
  3. The freedom to copy and share – WordPress can be downloaded and shared with and by anyone
  4. The freedom to modify – Anyone can download and modify WordPress, and distribute modified copies

Any work that is derived from core WordPress software or requires the core software to run, such as plugins or themes, inherits the GPL license. WordPress derivatives can only be distributed under the same terms as WordPress itself.

If a WordPress plugin or theme author doesn’t license their software as GPL, they limit the rights of every WordPress user that uses their product — which also breaks the WordPress license. People or companies that don’t embrace the WordPress license and protect the WordPress trademark are not eligible to organize, speak at, volunteer at, or sponsor official WordPress events, and their plugins/themes will not be listed in the official directories.

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