Are PostgreSQL clusters and servers the same thing?

From the official documentation (emphasis mine): [Chapter 22. Managing Databases](https://www.postgresql.org/docs/9.6/static/managing-databases.html) > **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). [Chapter 18. Server Setup and Operation -> 18.2. Creating a Database Cluster](https://www.postgresql.org/docs/9.6/static/creating-cluster.html) > **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?