NX bit / DEP
termsecurityprogramming
Stands for: no-execute bit / data execution prevention
Marking memory pages as either writable or executable, but never both, so injected data cannot run as code.
The hardware bit is NX or XD; Windows calls the policy DEP, and the general principle is W xor X. It ended the simplest form of exploitation, where an attacker wrote instructions into a buffer and jumped to them, and it is the reason attackers moved to reusing code already present and legitimately executable in the process. That escalation is the honest lesson of this whole family: each mitigation raised the cost substantially without ending the game, which is why they are deployed together rather than chosen between.
Also known as: DEP, data execution prevention, W^X, no-execute