An extension may refer to any of the following:
1. In general, an ext or extension is a part 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 (universal serial bus) extension cable, DVI/VGA extension cable, and a network extension cable. There is an extension cord for almost every cable ever made. However, realize 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. With a file name, a file extension, or file name extension, is the series of characters after a period that indicates the file type. For example, .exe is the file extension of executable files on Windows computers.
3. With 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.
5. With a phone or PBX (private branch extension), a ext or phone extension is an additional phone 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.