Animation is the art of using multiple images to depict motion/create the illusion of movement. Whereas a painting or picture only shows a static image, an animation is a series of successive static images/drawings/models that convey information about movement in the scene.
Our eyes are only able to retain images for about $100$ milliseconds thus when presented with many successive images, the viewer’s brain processes these images to appear as a single moving image.
Frame rate
This is the number of frames presented to the viewer per unit time (seconds). Modern cinema uses a frame rate of $24$ frames per second although certain productions use frame rates like $12$ frames per second to make the motion choppy e.g. Fantastic Mr. Fox:
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Television uses $30$ frames per second.