Every program that interacts with people responds to a specific set of commands. The set of commands and the syntax for entering them is called the user interface and varies from one program to another.
The DOS operating system makes a distinction between internal and external commands. Internal commands are commands, such as COPY and DIR, that can be handled by the COMMAND.COM program. External commands include the names of all other COM files, as well as EXE and BAT files.
Another word for command is directive.