A reticle is defined as a series of fine lines or fibers in the eyepiece of an optical device. Like what you see when you look into a rifle scope.

What are the different types of scope reticles?

What does Duplex reticle mean?

Duplex reticles go by different names depending on the manufacture, but they are all made up of a thin center wire with heavier sections on the edges. The distance between the intersection of the wires at the center, and the point where the wire thickens, can be used just like a ballistic aiming point.