If you've ever peered through a telescope, you've seen a reticule, though you probably didn't know that's what you were looking at. It's the network of tiny lines that make up a sighting device's eyepiece.
The crosshairs of a gun's scope or sight — which help a marksman aim when shooting — are one kind of reticule. Others exist, whether they can be clearly seen or not, in just about any instrument you look through, whether it's a telescope or a microscope. Another way to spell it is reticle, but either way it comes from the Latin word reticulatus, "having a net-like pattern," itself from reticulum, "little net."