A file extension is the last three or four characters after the period that make up an entire file name. The file extension helps the operating system determine which program on your computer the file is associated with.
In the file name myspreadsheet.xls, the file extension is xls.
This file extension happens to be associated with the Microsoft Excel software program. When you attempt to open this file, the xls file extension tells your operating system to open it with the Microsoft Excel program.
Also Known As: file suffix
Examples: "When I was renaming a music file I downloaded, I accidentally changed the file extension. When I tried to open it, my computer didn't know it was a music file anymore. Once I renamed it with the correct file extension, it worked fine."