The usual definition of process is: a running program

In order to switch the processor from one process to another, the operating system must be able to save the state of the current process and to restart it in the same state when it switches the processor back to this process.

What constitutes the state of a process?

• which instruction is next
• memory where the program code and data is located
• values stored in registers
• files it has open
• which process created it
• is it running or waiting for some resource
• ...