Definition - What does Flag mean?
A flag is one or more data bits used to store binary values as specific program structure indicators. A flag is a component of a programming language's data structure.
A computer interprets a flag value in relative terms or based on the data structure presented during processing, and uses the flag to mark a specific data structure. Thus, the flag value directly impacts the processing outcome.
Techopedia explains Flag
A flag reveals whether a data structure is in a possible state range and may indicate a bit field attribute, which is often permission-related. A microprocessor has multiple state registers that store multiple flag values that serve as possible post-processing condition indicators such as arithmetic overflow.
The command line switch is a common flag format in which a parser option is set at the beginning of a command line program. Then, switches are translated into flags during program processing.