cross hair

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

• noun Either of two fine strands of wire crossed in the focus of the eyepiece of an optical instrument and used as a calibration or sighting reference.

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

• noun either of two fine mutually perpendicular lines that cross in the focus plane of an optical instrument and are use for sighting or calibration