The Free On-line Dictionary of Computing (30 December 2018):
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