From PostgreSQL wiki
PostgreSQL versus Postgres
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. Per Dave Page, PostgreSQL Core Team:
Following recent discussions on a name change for the project, it has become clear that consensus within the community will never be reached. In light of this, the core team have discussed the matter in depth and decided that the project shall continue to use the name PostgreSQL, but accept the use of Postgres as an alias.