Arcs.

Arcs in animation is where objects follow a realistic, arced trajectory to give a greater sense of realism. This seems like a very obvious and automatic principal, however it can sometimes be done wrong, or not done at all.

A good example of arcs can be seen in Gears of War 3.

This screen grab of Gears of War 3 gameplay shows the guide path of a grenade, this is the exact path the grenade travels, showing an arced trajectory which looks like it would be fairly realistic, this trajectory also changes in height and distance depending on where you aim it.

On the opposite side of the spectrum, here is a bad example of arcs from Mass Effect.

Although not overly clear from this image, it shows the player having just thrown a grenade, in the image, the grenade can be seen just by the enemies knee. Grenades in Mass Effect of no trajectory whatsoever, meaning if the player aimed at the horizon, the grenade would keep going until it hit something or automatically exploded after a long time.