(1) In communications, a code in the transmitted message which indicates that the following characters are a control code and not data.

(2) In programming, a "yes/no" indicator used to represent the current status of something. A flag is often only one bit of the byte and is created and controlled by the programmer in software. When only a single bit is used, eight flags, or status conditions, can be represented by one byte. Flags can also be built into and turned on and off by hardware, in which case the software is used only to read the flag to determine the current condition of the device.

(3) A Unix command line argument. The symbol is a dash. For example, in the command head -15 filex, which prints the first 15 lines of the file FILEX, the -15 flag modifies the Head command.

(4) To identify an element of data or a process by embedding a code (flag) in it.