A pen is a long thin object which you use to write in ink.

If someone pens a letter, article, or book, they write it.

A pen is also a small area with a fence round it in which farm animals are kept for a short time.

If people or animals are penned somewhere or are penned up, they are forced to remain in a very small area.

People sometimes say the pen to refer to a prison.

If you put pen to paper, you write something.