1.NounA shell, a command line interface; usually used with the definite article.
"I'm so used to the command line that I don't even know how to find things in the GUI.; The command line offers a lot of helpful features, such as tab completion and wildcard expansion."
2.NounThe text prompt presented to the user in a command line interface.
"Type the following at the command line: du -s -h."
3.NounA line of text that is entered at such a prompt.
"Use the following command line to compile a file without linking: gcc -c main.c -o main.o"