Is Tuesday named for a one-handed god?
A program which reads textual commands from the user or from a file and executes them. Some commands may be executed directly within the interpreter itself (e.g. setting variables or control constructs), others may cause it to load and execute other files.
Unix's command interpreters are known as shells.
When an IBM PC is booted BIOS loads and runs the MS-DOS command interpreter into memory from file COMMAND.COM found on a floppy disk or hard disk drive. The commands that COMMAND.COM recognizes (e.g. COPY, DIR, PRN) are called internal commands, in contrast to external commands which are executable files.