The NX bit (no-execute) is a technology used in CPUs to segregate areas of a virtual address space for use by either storage of processor instructions or for storage of data. An operating system with support for the NX bit may mark certain areas of an address space as non-executable. The processor will then refuse to execute any code residing in these areas of the address space. The general technique, known as executable space protection, also called Write XOR Execute, is used to prevent certain types of malicious software from taking over computers by inserting their code into another program's data storage area and running their own code from within this section; one class of such attacks is known as the buffer overflow attack.

The term NX bit originated with Advanced Micro Devices (AMD), as a marketing term. Intel markets the feature as the XD bit (execute disable). The MIPS architecture refers to the feature as XI bit (execute inhibit). The ARM architecture refers to the feature, which was introduced in ARMv6, as XN (execute never). The term NX bit itself is sometimes used to describe similar technologies in other processors.