WordReference Random House Unabridged Dictionary of American English © 2020
cross hairs′, 
  1. OpticsAlso called  cross wires. fine wires or fibers, strands of spider web, or the like, crossing in a focal plane of an optical instrument to center a target or object or to define a line of sight.
  2. Printingmarks for aiding in the positioning of overlaying images for correct registration in printing. Cf. register (def. 11b).
  • 1880–85

Collins Concise English Dictionary © HarperCollins Publishers::
cross hairs pl n
  1. 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: cross wires

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