Question

What are harmonics? Which characteristic of a sound do they determine?

Answer 1

Harmonics -  a component frequency of the signal of a wave that is an integer multiple of the fundamental frequency

Answer 2

In music, harmonics are used on string instruments and wind instruments as a way of producing sound on the instrument, particularly to play higher notes and, with strings, obtain notes that have a unique sound quality or "tone colour". On strings, harmonics that are bowed have a "glassy", pure tone.