What is the definition of dominance in fine art?

Dominance is not Prominence or Contrast. Dominance has to do with how much area is covered by the characteristics of Shape, Direction, Color, Color, Value, Texture and Size. For example, a painting may have a gray dominance, but hold a bright, saturated color for contrast to hold the eye at the center of interest. Dominance is necessary for contrast and unity and is achieved through repetition and/or size.

For a painting to have "Shape Dominance" one need only look at the character of the shapes which overwhelm the picture space, be they geometric, or organic.

While unity is the goal to achieve in a painting (or novel, or poem, or music) a dominant theme must be present, which lends itself to the outcome in overall mood (or emotional content) of the art.