Harmony is broadly understood to involve both a "vertical" dimension (frequency-space) and a "horizontal" dimension (time-space), and often overlaps with related musical concepts such as melody, timbre, and form.
1. agreement; accord; harmonious relations. 2. a consistent, orderly, or pleasing arrangement of parts; congruity. 3. a. any simultaneous combination of tones. b. the simultaneous combination of tones, esp. when blended into chords pleasing to the ear; chordal structure, as distinguished from melody and rhythm.
When people are in harmony or in harmony with each other, they live together in a peaceful and friendly way. She lives in harmony with her neighbors. To bring something into harmony with something else is to change it so that it agrees with or matches something else.