All versions of PostgreSQL. Add an additional version-specific tag like [postgresql-13] if that context is relevant.

PostgreSQL is a powerful, open source ACID compliant MVCC database system with a proven architecture that has earned a strong reputation for standards-compliance, reliability, data integrity, and extensibility.

PostgreSQL runs on all major operating systems and supports most of the major features of SQL:2016 including foreign keys, transactions, and window functions, views and materialized views, triggers, and CTEs (including recursive CTEs).
Functions and procedures can be written in multiple languages including C, PL/pgSQL, Python, Perl, TCL, and numerous other languages including Java, Lua, R, sh, and v8 ECMAScript/JavaScript.

Types

PostgreSQL supports most of the standard SQL types including:

Indexes

PostgreSQL supports multicolumn, unique, full, partial and functional indexes, and combinations thereof. Index types include B-tree, Hash, GiST, SP-GiST, GIN and BRIN.

Replication and High Availability

PostgreSQL supports both logical replication, and WAL-shipping.

Extensions

A multitude of extensions provides additional capabilities. Notably, the spatial extension PostGIS is the best in the industry. See also

License

PostgreSQL is released under the PostgreSQL License, a liberal Open Source license, similar to the BSD or MIT licenses.