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The following definitions are from the Artlex Art Dictionary. Click the link for a more extensive description of these terms.
Rhythm - A visual tempo or beat. The principle of design that refers to a regular repetition of elements of art to produce the look and feel of movement. It is often achieved through the careful placement of repeated components which invite the viewer's eye to jump rapidly or glide smoothly from one to the next.
Composition- The plan, placement or arrangement of the elements of art in a work. It is often useful to discuss these in reference to the principles of design, as well as to the relative weight of the composition's parts.
Harmony - Agreement; accord. A union or blend of aesthetically compatible components. A composition is harmonious when the interrelationships between its parts fulfill aesthetic requisites or are mutually beneficial. As a principle of design, harmony refers to a way of combining elements of art to accent their similarities and bind the picture parts into a whole. It is often achieved through the use of repetition and simplicity.
Elements of art or elements of design - The basic components used by the artist when producing works of art. Those elements are color, value, line, shape, form, texture, and space. The elements of art are among the literal qualities found in any artwork.