PostgreSQL, not Postgres.

FWIW: "The name Postgres is an accepted alias for the PostgreSQL project. However it is an alias or nickname and is not the official name of the project."

https://wiki.postgresql.org/wiki/ProjectName

Well, it's both: the original name was Postgres, later extended to include SQL in it.

It was Postgres for the first ten years since the start in 1985, but it’s been “PostgreSQL” for more than twenty years now.