From the official documentation (emphasis mine):
Every instance of a running PostgreSQL server manages one or more databases. Databases are therefore the topmost hierarchical level for organizing SQL objects ("database objects").
Chapter 22. Managing Databases -> 22.1 Overview
More accurately, a database is a collection of schemas and the schemas contain the tables, functions, etc. So the full hierarchy is: server, database, schema, table (or some other kind of object, such as a function).
A database cluster is a collection of databases that is managed by a single instance of a running database server.
I don't quite understand the difference between the PostgreSQL concepts of a cluster and a server, but it seems to me that they are the same thing?