harmony -
a harmonious state of things in general and of their properties (as of colors and sounds); congruity of parts with one another and with the whole
harmony -
The relationship between two distinct musical pitches musical pitches being frequencies of vibration which produce audible sound played simultaneously.
Wikipedia
Harmony is the use of simultaneous pitches, or chords. The study of harmony involves chords and their construction and chord progressions and the principles of connection that govern them. Harmony is often said to refer to the "vertical" aspect of music, as distinguished from melodic line, or the "horizontal" aspect.
OmegaWiki Dictionary
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harmony A pleasing combination of elements, or arrangement of sounds.
harmony Two or more musical notes played simultaneously to produce a chord.