Other forms: pens; penned; penning
A pen is an ink-filled tool that you use for writing or drawing. Doing a crossword puzzle with a pen shows a lot of self confidence, since you can't erase your answers.
Pen is also a verb meaning "to write," especially when you're writing something of literary value: "She's my favorite writer — I can't wait for her to pen another novel!" Then there's the pen that you keep your pet donkey safely inside (you can also say you pen him in there), and inspired by this "livestock enclosure" meaning there's also the slang for "penitentiary" or "jail."