The command line is a textual prompt for a command.
It's also used to describe the textual interface to the system, as in, "I dropped to the command line and ran the application."
And actually most people just say Command Line, when they mean typing something into a Unix Shell or Microsoft Windows command or cmd window.
There's an interesting sense of height when referring to the command line, as illustrated in the quote above. There's a sense that the command line is "lower" in the system, closer to the base of the system.
See original on c2.com 