Harmony is abiding in agreement and in one accord. It is a union or blend of aesthetically compatible components in a work of art.
The composition is harmonious when the interrelationship between its elements fulfill the prescribed aesthetic requisites in order to express what was in the mind of the artist. As a principle of design, harmony refers to a way of combining elements of art to accent their similarities and bind the parts into a whole. Harmony is often achieved through the use of repetition and conscious simplicity. However, excessive harmony leads to monotony, coldness, and boredom. Relieving this may be through the deft use of elements like contrast, chiaroscuro, perspective or strategic dissonance.