Also called: postmaster process, supervisor process, parent postgres process
In plain English
The postmaster is the supervisor process that starts a PostgreSQL server. It creates the shared memory segment, opens the listening socket, forks a dedicated backend for each client connection, launches the background worker processes, and monitors all its children. Every other PostgreSQL process is its child.
Why it matters
The postmaster is also the crash manager: if a backend dies unsafely, it resets shared memory and restarts every child to guarantee consistency — which is why a single backend crash briefly drops all connections. It never touches your data directly; it only supervises.