a process

Think of a recipe printed in a cookbook versus the actual cooking happening in a kitchen right now. The recipe is just paper - the same recipe can be cooked many times. A process is the cooking: one live, running instance of a program, with its own pots on the stove, its own half-chopped onions, its own place in the steps. The cookbook (the program file on disk) does nothing by itself; a process is what happens when the operating system picks it up and starts running it.

Concretely, when you launch a program the operating system creates a process for it: it carves out a private region of memory holding the program's code, its data, its stack, and its heap; it gives the process a number to identify it; and it starts the CPU executing the program's instructions on its behalf. The kernel keeps a little record (a process descriptor) tracking everything about that running instance - what memory it owns, which files it has open, whether it is currently running or waiting, and who its parent is. The process is that whole bundle of running state, not the file it came from.

Why it matters: the process is the operating system's fundamental unit of 'a program that is running'. The OS schedules CPU time per process, hands out memory per process, and isolates one process from another so a crash in one does not corrupt the others. Almost everything else in this field - creating processes, ending them, finding out how they finished - is about managing these running instances. Keep the distinction sharp: a program is a file that sits still; a process is that file brought to life and given resources to run.