Animation is an object that moves across or into or out of the screen.

Persistence of vision is what allows people to see animation. The human eye will still hold an image for a brief moment. The brain seeing a series of images change over time will see movement. Flip books& Zoetropes let you see a series of moving images on paper as an animation.

Animation & Multimedia Sequencing:
Animation is when the computer shows a series of movements of objects.
Frame animation has a series of graphics that are shown in rapidly on the screen. Sort of like a flip book animation.
Cel animation. You have a series of keyframes and then you draw the in between frames. Such as a character walking. Tweening fill in the keyframes. This can be done by hand, as Disney did years ago. However with a computer program, it can be sophisticated enough to calculate the in between frames. In cels, the parts of the scene are moved about. So a person's mouth is on a different cel or layer. To change the mouth as they speak, you only need to update a tiny portion of the image.