What does "harmonics" mean?

Harmonics: pl. (Mus.) Secondary and less distinct tones which accompany any principal, and apparently simple, tone, as the octave, the twelfth, the fifteenth, and the seventeenth. The name is also applied to the artificial tones produced by a string or column of air, when the impulse given to it suffices only to make a part of the string or column vibrate; overtones.