A harmonic is a whole-number multiple of the fundamental frequency of a sound. It is an element of the harmonic series.
The timbre of harmonic oscillators, such as a bowed violin or the human voice, contains a nearly infinite amount of harmonic partials, starting with 1f, 2f, 3f, 4f... where f is the fundamental frequency. Each of these harmonics has a distinct amplitude, generally decreasing as the 'height' of the harmonic increases. The span between any two of these harmonics is called a just interval.
A subharmonic is a unit fraction of the fundamental frequency of a sound. It is an element of the subharmonic series.