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cross hairor cross·hair (krôs′hâr′, krŏs′-)
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.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(General Engineering) either of the two fine mutually perpendicular lines or wires that cross in the focal plane of a theodolite, gunsight, or other optical instrument and are used to define the line of sight. Also called: crosswire
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014