An enclosure (enclosed area) used to contain domesticated animals, especially sheep or cattle.
There are two steers in the third pen.
A prison cell.
They caught him with a stolen horse, and he wound up in the pen again.
The bullpen.
Two righties are up in the pen.
To enclose in a pen.
A tool, originally made from a feather but now usually a small tubular instrument, containing ink used to write or make marks.
He took notes with a pen.
A writer, or his style.
He has a sharp pen.
Marks of ink left by a pen.
He's unhappy because he got pen on his new shirt.
A light pen.
The internal cartilage skeleton of a squid, shaped like a pen.
A feather, especially one of the flight feathers of a bird, angel etc.
A wing.
To write (an article, a book, etc.).
A female swan.
Penalty.