PostgreSQL is an object-relational database management system (ORDBMS). It is released under a BSD-style license and is thus free software. As with many other open-source programs, PostgreSQL is not controlled by any single company, but has a global community of developers and companies to develop it.

The mixed-capitalization of the PostgreSQL name can confuse some people on first viewing. The several pronunciations of "SQL" can lead to this confusion. PostgreSQL's developers pronounce it /poːst ɡɹɛs kjuː ɛl/; (Audio sample, 5.6k MP3).

It is also common to hear it abbreviated as simply "Postgres", which was its original name. Because of ubiquitous support for the SQL Standard amongst most relational databases, the community considered changing the name back to Postgres. However, the PostgreSQL Core Team announced in 2007 that the product would continue to be named PostgreSQL.

The name refers to the project's origins as a "post-Ingres" database, the original authors having also developed the Ingres database. PostgreSQL evolved from the Ingres project at University of California, Berkeley. In 1982, the project leader, Michael Stonebraker, left Berkeley to commercialize Ingres.

He returned to Berkeley in 1985 and started a post-Ingres project to address the problems with contemporary database systems that had become increasingly clear during the early 1980s. The new project, Postgres, aimed to add the fewest features needed to completely support types. These features included the ability to define types and to fully describe relationships – something used widely before but maintained entirely by the user.

In Postgres, the database "understood" relationships, and could retrieve information in related tables in a natural way using rules. Postgres used many ideas of Ingres but not its code.

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