writable or executable, but not both. Without such protection, a program can write (as data "W") CPU instructions in an area of memory intended for data and then run (as executable "X"; or read-execute "RX") those instructions. This can be dangerous if the writer of the memory is malicious.

The terminology was first introduced in 2003 for Unix-like systems, but is today also used by some multi-platform systems (such as .NET). Other operating