A flag may refer to any of the following:
1. A flag is bit of information that can have an ON or OFF value, more commonly represented as 1 or 0. For example, in the win.ini on an IBM compatible computer running Microsoft Windows a flag to set the wallpaper to tiled on is: TileWallpaper=1.
2. A flag is also a mark that indicates a certain event has taken place, or that an object is unusual in some way. For example, in the world of computer programming, and error flag indicates that some or all the code is not working properly.
3. In an e-mail program, a flag is a visual reminder of an important e-mail. For example, as you are going through e-mail important messages can be flagged to be dealt with later.