DEP (Data Execution Prevention) is a security feature in modern operating systems that prevents code from executing in memory regions designated for data only. DEP marks certain memory pages as non-executable (NX bit).
This mitigates an entire class of exploits: buffer-overflow attacks that inject shellcode into the stack or heap fail because the processor refuses execution. DEP has been available since Windows XP SP2 and in all current Linux kernels. Together with ASLR (Address Space Layout Randomization), DEP forms the foundation of modern OS-level exploit protection.