The 12 principles of animation number 4#: arcs

an arc is the edge of a section of a circle, if you were to follow the arc it would create a full circle. An example of this in reality is pendulum.
a good example is in kingdom hearts when the protagonist (Sora)  attacks with his keyblade, he uses an arc movement to allow his attack the furthest reach in his field of vision. this also is a good example if anticipation.
bad example: Tetris because when the pieces are being rotated they don't arc they just turn but by not arcing it does not waste time in game play when the pieces are falling down fast.