A principle of art, unity can be defined as similarity, oneness, togetherness, or cohesion. Variety can be defined as difference. Unity and Variety are the cornerstones of composition. When they are combined effectively, we can create compositions that are both cohesive and lively. (Grouping, Containment, Repetition, Proximity, Closure, Combining Gestalt Principles).
In design, balance refers to the distribution of weight or force within a composition. (actual balance and pictorial balance, symmetrical balance, asymmetrical balance, horizontal, vertical, diagonal and radial balance, imbalance).
Emphasis gives prominence to part of a design. A focal point is a compositional device used to create emphasis. Both emphasis and focal point are used to attract attention and increase visual and conceptual impact. (emphasis by Isolation, emphasis by Placement, emphasis through Contrast).
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.
Refers to the size of a form when compared with our own human size. (hierarchical scale, distortion of scale).
Is a principle of art that describes the size, location or amount of one element to another (or to the whole) in a work. It has a great deal to do with the overall harmony of an individual piece.
Reed Enger, "The Elements of Design," in Obelisk Art History, Published August 06, 2019; last modified September 04, 2020, http://arthistoryproject.com/essays/the-elements-of-design/.