My Definition: The presence of multiple pitches occurring simultaneously.
Just like melody, harmony isn't necessarily always a good thing. It is simply the occurrence of two or more notes at the same time. It's multiple musical sounds simultaneously. Also like with melody, harmony can also be pleasant or unpleasant. Pleasant harmonies, when the notes sound nice and sweet together, it's called consonant. Unpleasant harmonies, when the notes clash and don't go well together, it's called dissonant.
Vivaldi's "Spring" is a lovely example of a consonant harmony.
Wagner's "Die Meistersinger von Nurnberg" has a lot of dissonance in it.
