Unity can be defined as the similarity, oneness, togetherness, or cohesion of the elements within a composition. Variety is unity’s opposite—the difference between elements. Unity and Variety are the cornerstones of composition, and when combined, can create compositions that are both cohesive and lively. Unity and variety can be explored through techniques like:
Balance refers to the distribution of weight or force within a composition.
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.
Rhythm is 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.
Scale refers to the size of a form when compared with our own human size.
Proportion describes the size, location or amount of one element to another or to the whole in a work of art.
Reed Enger, "The Principles of Design," in Obelisk Art History, Published August 06, 2019; last modified September 16, 2021, http://arthistoryproject.com/essays/the-elements-of-design/.