light pen

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small, photosensitive device connected to a computer and moved by hand over an output display in order to manipulate information in the computer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computing A pen-like device allowing a computer user to interact with the screen display.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (computer science) a pointer that when pointed at a computer display senses whether or not the spot is illuminated