It's a particular program while it is being executed and managed by the operating system
The process can be terminated (or "killed") by you, or by the operating system. At that point, it stops running/being executed, and it can no longer do things.
There can be multiple processes of the same program running at the same time.
Workers
Multiple processes can run within the same application at the same time. Those multiple processes are commonly called workers
Example: a http server that listens on a single port (manager process), but then transmits the communication to each worker process
Multiple processes normally do not share any memory