Reserved word

1. A word within a program or programming language that is reserved for special use that cannot be used. Reserved words are often found in programming languages and macros.

2. Reserved words are also used in operating systems as a method of identifying a device file or other service. For example, below is a listing of Microsoft reserved words in MS-DOS and Microsoft Windows operating systems. When attempting to use any of the below reserved words as a name of a file, or in a command you may encounter and unusual response. For example, attempting to save a file as CON or CON.txt may generate a reserved file name error, may say the file already exists, or may cause an access denied error. Because these are reserved words you cannot create a file using any of the examples below.

Reserved Word What it is
AUX Auxiliary port aka Serial Port COM1
CON Short for console, which Microsoft describes as the display monitor.
COM1 COM port.
COM2 COM port.
COM3 COM port.
COM4 COM port.
LPT1 LPT port.
LPT2 LPT port.
LPT3 LPT port.
NUL NULL
PRN Printer aka LPT1

Also see: Device file, Java reserved words, Operating system terms, Programming terms, Reserved character, Word