PostgreSQL is 100% open source software.
PostgreSQL is freely available to use, alter, or redistribute in any way you choose. PostgreSQL's license is an approved open source license very similar to the BSD (Berkeley Distribution Software) license, though only just different enough that it is now known as TPL (The PostgreSQL License).
PostgreSQL is already in use by many different application packages, and so you may already find it installed on your servers. Many Linux distributions include PostgreSQL as part of the basic installation, or include it with the installation disk.
One thing to be wary of is that the version of PostgreSQL included may not be the latest release. It will typically be the latest major ...