Reticle: “A grid or other pattern of fine threads, wires, lines, etc., in the focal plane or eyepiece of a telescope or other optical instrument in order to facilitate positioning, aiming, and measurement.” (Oxford English Dictionary) Synonymous with “crosshairs.” From Latin reticulum, a small meshwork bag, a diminutive of rete “net.” (Online Etymology Dictionary)
The OED says reticule is a synonym for reticle, but reticule more commonly refers to a mesh bag.
Reticles, from Wikipedia.
1920s reticule, from VictorianElegance.com
In the language of guns and shooting, a reticle is “the crosshair or pattern placed in the eyepiece of the scope which establishes the gun’s position on the target” (from the Bushnell website).
Reticles are also in other sighting devices, including telescopes, microscopes, and surveying instruments.
“Reticle” is usually a bit of specialized jargon, but it gained more widespread attention after the Saturday-morning shootings of Arizona Congresswoman Gabrielle Giffords, a federal judge, and at least 18 other people. Shortly after the first reports of the shootings, many bloggers and Twitter users linked to a map that media personality Sarah Palin had posted on her Facebook page in March 2010. The map showed reticles—either gun sights or, as Palin’s spokeswoman said Saturday afternoon, “surveyor’s symbols”—on 20 congressional districts, including that of Rep. Giffords, who was mentioned by name on the map. A November 4, 2010, tweet from Palin herself (or whoever tweets on her behalf) referred to to the icons as “bullseyes,” with no mention of surveying. The site on which the map appeared, Take Back the 20, was taken down Saturday afternoon.
In a March 2010 interview with MSNBC after her Tucson office was vandalized, Rep. Giffords said the icon was “the crosshairs of a gun sight.”
On Sunday afternoon, a Google search for <reticle Giffords> produced more than 81,000 matches.