From Halopedia, the Halo wiki
A reticle, alternatively spelled reticule, is a shape superimposed on an image that is used for precise alignment of a device. Crosshairs are most commonly represented as intersecting lines in a "+" shape, though many variations exist, including dots, posts, circles, scales, or chevrons. The reticle is the primary means by which players (and Spartans, Marines, and Elites) aim their weapons. It is located in the center of the screen and changes based on which weapon the person is wielding.