Crosshairs are the wires that are lined up when a shooter is aiming a gun. A hunter might get a moose in her crosshairs before taking a shot.
Crosshairs, also spelled as two words, cross hairs, is another term for a reticle, the series of lines in an optical piece used to bring an image into focus. Binoculars, telescopes, and microscopes all use the same kind of device for focusing as the crosshairs of a rifle or gun. The word crosshairs simply comes from the two lines that intersect in the shape of a cross.