An easily noticed or understood difference between two or more things. In art, contrast is the difference between dark and light colors or dark and light areas.
Contrast is a strategy used by an artist to break up a work of art. In many ways, contrast is the opposite of the element of unity, in that it commands the viewer's attention by sheer force of its differences.
Contrasts can be paired colors which are chromatic opposites: in a work strictly adhering to unity those colors would be complementary.
Contrast can also be antagonist colors and shapes: Renaissance painters like Rembrandt and Caravaggio used the contrasting technique known as chiaroscuro. These artists set their subjects in a darkly lit room but picked them out with a single pool of contrasting light. In these types of uses, contrast does not express parallel ideas, but rather, sets aside the subject as unique or significant or even sanctified compared to its background.
Consider Jackson Pollack's canvases, which are extremely chaotic and laid down in contrasting lines and blobs of color, but the end effect is rhythmic in composition and unified in all of its variety. So, in effect, unity and contrast are two ends of a scale. The overall effect of a composition located near the variety/contrast end would be described as "interesting," "exciting," and "unique."