Definition
Plain language
Keeping each running program walled off in its own space so one can't touch another's data or crash its neighbor.
As stated in the literature
An operating-system and security mechanism giving each process its own protected memory and resource boundaries; used as a defense layer so a compromised or misbehaving component can't reach into others, e.g. one layer of a hardened reward verifier against agent tampering.
Why it matters: It contains damage so a broken or malicious component can't reach into or crash the rest of the system.
For example, if one program in your web browser crashes, process isolation keeps it from taking down the other tabs or reading their data.