A Crosshair is a weapon feature introduced when the game came out. It serves as an indicator for the current direction the player is aiming at, as well as a guide for the current accuracy and spread of a weapon.
In many weapons, it appears to be a white cross with a separated dot on the center. Crosshairs can come into a wide variety of forms, especially in clan weapons, but the "cross" form is the most common one.
When a weapon is fired, the crosshairs will spread away from the center of the screen. The longer the player shoots, the more the crosshair spreads out and the wider the bullet spread will be. The crosshair will reset to its initial position when reloading. Different weapons can have different types of crosshairs. For example, shotgun type weapons have a circular crosshair indicating the spread of their shots, while Heavy weapons such as rocket launchers have a square crosshair.
Crosshairs were a base addition to the game. Though, they have evolved over time. The first crosshair design had a circle with another, smaller circle in the middle of the larger one. 4 triangles were the interior of the smaller circle's edge, evenly spaced, with a '+' like shape in the middle, connecting the four triangles.(presumably) From the 2.7.0 update to the 10.2.1 patch, all weapon crosshairs would consist of 4 rectangles, and nothing in-between. From the earliest version to 2.7.0, it used to have a crosshair similar to Third Eyes new crosshair, then in a unkown update, it was changed to a more generic crosshair seen in other first person shooters, then after 10.2.1, all weapons now have unique crosshairs. (Melee weapons, the Battle Mech, Demon Stone, and Robot Samurai Gadgets had the old generic crosshair, however Melee weapons got their crosshairs changed a short while after)