Definition:
ASLR, or Address Space Layout Randomization, is method in which key data areas in the address space are randomized, making it more difficult for attackers to guess the location of libraries needed to carry out heap and stack based attacks.
ASLR has been present in Linux-based operating systems for some time, but was first introduced in Microsoft operating systems beginning with Windows Vista. Internet Explorer 8 running on Vista, Server 2008, or Windows 7 also supports ASLR.