a process

Think of a cookbook on a shelf. The book is just paper and ink until someone takes it into the kitchen, gathers the ingredients, and actually starts cooking. The book is the same for everyone, but each cook in each kitchen has their own pots, their own half-chopped onions, their own place in the recipe. A process is a recipe being cooked. It is a program that has been brought to life and is actually running, with all the in-progress state that running requires.

More precisely, a process is a program in execution together with everything the operating system must track to keep it running: its own region of memory (its address space, holding the code, the data, the heap, and the stack), the current values in the CPU registers including the program counter that says which instruction comes next, a list of files it has open, and bookkeeping such as how much CPU time it has used. When you double-click an application, the OS creates a process: it loads the program's code into memory, sets up a fresh address space, and starts the CPU executing from the program's entry point.

The key idea is that a process is the unit the OS schedules onto the CPU and protects from other processes. Each process believes it has the machine to itself, even though dozens or hundreds run together by taking quick turns. One important and common confusion: a process is not the same thing as a program. The program is the passive file on disk; the process is one active, running instance of it, and the same program can be running as many separate processes at once.