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Crosshairs are thin wires or lines serving as focusing tools on gun sights, but did you know that the etymology of the name is in fact quite literal? The story traces back to seventeenth century, when the noted polymath Robert Hooke crossed two hairs on the lens of a telescope to make the view more precise. By 1755, crosshair was adopted to describe this, for obvious reasons, and the word in reference to gun scopes was first coined in 1785. Eventually, the actual crossed hairs were phased out for built-in or computer-projected markings, but the word remained, hinting at the more rudimentary origins. Something very interesting in the history of the word is that usage jumped an enormous eightfold between the years 1980 and 2000, according to Google NGrams. There are no explanations for this online, but I'm guessing that it has something to do with the increasing prevalence of crosshairs in popular culture, particularly due to recent video games and action movies. Other words for crosshair include reticle, reticule, graticule, and, metonymically, sight and scope work too.