(noun) - Rhythm is a principle of art that's difficult to summarize in words. Assuming that you've picked up on a rhythm in music before, take what you heard with your ears and try to translate that to something you'd see with your eyes. Rhythm, in art, is a visual beat.
A pattern has rhythm, but not all rhythm is patterned. For example, the colors of a piece can convey rhythm, by making your eyes travel from one component to another. Lines can produce rhythm by implying movement. Forms, too, can cause rhythm by the ways in which they're placed one next to the other.
Really, it's easier to "see" rhythm in just about anything other than the visual arts. Literally-minded types should stick to music for rhythm.