A system can resonate at many frequencies. The lowest frequency at which a system can resonate is called the fundamental frequency. Resonant frequencies higher than the fundamental frequency are called overtones. The second highest resonant frequency is called the 1 st overtone, the 3 rd highest the 2 nd overtone, etc. The fundamental frequency is the 1 st harmonic.
Higher harmonics are the other resonant frequencies.
For example, the 2 nd harmonic has a frequency that is twice the fundamental, the 3 rd harmonic has a frequency that is three times the fundamental frequency, etc.