* Adjective: This is the most common usage, describing something that is related to harmony, or that creates or is characterized by harmony. For example, "The music had a beautiful harmonic sound."
* Noun: It can refer to a single component in a musical chord, or to a set of musical tones that sound pleasing together. For example, "The first harmonic in the series is the fundamental frequency."
* Verb: It can be used to describe the act of creating or playing harmonious sounds. For example, "The singers harmonized beautifully."
To determine the correct part of speech, you need to look at the sentence it's used in and consider how it's functioning.