Let’s consult the dictionary first. It’s dry as dust, but we’ll go with it for now:

Harmony /här′mə-nē/
noun

An orderly or pleasing combination of elements in a whole.
“color harmony; the order and harmony of the universe.”

True dat. But it’s only a good start. Let’s keep going. It has a more cogent definition in music:

In music, harmony is the concept of combining different sounds in order to create new, distinct musical ideas. Theories of harmony seek to describe or explain the effects created by distinct pitches or tones coinciding with one another; harmonic objects such as chords, textures and tonalities are identified, defined, and categorized in the development of these theories. Harmony is broadly understood to involve both a “vertical” dimension and a “horizontal” dimension, and often overlaps with related musical concepts such as melody, timbre, and form.

Yikes. That’s like Health Class in high school, trying to explain getting jiggy wid it. Doesn’t do justice to the topic.,

Oh, the hell with it:

Harmony /här′mə-nē/ noun. See: The Quebe Sisters.