Pronounced ann-see-dot -siss, the name of a DOS device driver that makes a monitor conform to the ANSI standard. The ANSI standard specifies a series of escape sequences that cause the monitor to behave in various ways. For example, one escape sequence clears the screen while another causes all subsequent characters to be inverted.
In general, DOS programs do not use the ANSI codes because these codes are slower than the built-in BIOS codes. DOS programs that do use ANSI codes for compatibility with other devices require that you load the ANSI.SYS device driver in the configuration file, CONFIG.SYS.
Windows does not use ANSI.SYS at all.