An extension may refer to any of the following:
1. In general, an extension is a part that is added to something that extends or enlarges it. For example, an extension cable or extension cord is a cord capable of connecting to another cord that extends its overall length regarding computer cables. A common example of extension cables used with a computer is a power extension cord, USB extension cable, DVI/VGA extension cable, and a network extension cable. There is an extension cord for almost every cable ever made. However, keep in mind that most cables also have length limitations. Most specialty computer stores and online retailers have extension cords that can be purchased at various lengths.
2. When referring to a file name, a file extension, or file name extension, is the series of characters after a period which indicates the file type. For example, .exe is the file extension of executable files on Windows computers.
3. When referring to an Apple Macintosh computer, extensions or system extensions are files stored in the extension folder that load into memory when the computer first loads. Extensions were known as an INIT on System 6 and earlier.
4. When referring to an Internet browser, an extension is synonymous with add-on.
5. With a phone or PBX, a phone extension is an additional phone that's connected to the same telephone line. For example, in an office, an employee may have a phone extension that allows someone to reach them directly when calling. A company could have one phone number with dozens or hundreds of extensions.