Definition of extension

A program or program module that adds functionality to or extends the effectiveness of a program.

The part of a PC filename after the dot (.), often used to tell Windows what type of file it is. For example, files ending in .exe are programs, and files ending .jpg or .gif are pictures. Note that Windows is often set up not to display the extension in Windows Explorer or My Computer, another helpful idea to confuse beginners from our friends at Microsoft.