bash - the programming language used in the command line. (Yes, we tricked you, you're already programming!) Short for "Born Again SHell," for reasons people on the internet will happily tell you about.
the terminal - Particularly used to refer to the command line on OSX. This term made more sense when universities used mainframes and every computer was only a terminal.
the shell - The part of an operating system that interacts with a human. Technically, anything you do in a graphical interface is also in a shell, but in practice this is just another synonym for the command line.
cli - "Command Language Interpreter," this is a super technical term for the command line used to impress everyone around you.
GUI - "Graphical User Interface." Pronounced "gooey," like delicious gooey chocolate. Basically, anything on a computer that isn't in the command line. All familiar elements of day-to-day computer tasks such as images, windows, prompts, buttons, and progress bars are part of the GUI. The way most people interact with computers. Some tasks can only be done in a GUI, while others can only be done in the command line.
root - A word for the administrative user on a system. You often need administrative privileges to install programs or access certain system folders using the command line. You can tell you're root when your $ prompt turns into a # prompt. To become root, type su and enter the password you use to log in. (No characters or asterisks will appear, just type your password and press enter.) You can also run a single command as root by typing sudo before the command.
UNIX - A family of operating systems that have a multi-user model and a particular design philosophy. Both OSX and Linux are UNIXes. Windows is not.
REPL - "Read Eval Print Loop" The process of typing something in to the command line and getting something back out. Like most things to do with the command line, not as complicated (or scary) as it sounds.