Every Supabase project comes with a full Postgres database, a free and open source database which is considered one of the world's most stable and advanced databases.
The database was derived from the POSTGRES Project, a package written at the University of California at Berkeley in 1986. This package included a query language called "PostQUEL".
In 1994, Postgres95 was built on top of POSTGRES code, adding an SQL language interpreter as a replacement for PostQUEL. Eventually, Postgres95 was renamed to PostgreSQL to reflect the SQL query capability.
After this, many people refered to it as Postgres since it's less prone to confusion. Supabase is all about simplicity, so we also refer to it as Postgres.