Final answer:
The lowest sound of an overtone series is called the fundamental, which is also referred to as the first harmonic.
Step-by-step explanation:
The lowest sound of an overtone series is called the fundamental. In any resonance system, such as an air column or string, the resonances occur at frequencies that are integer multiples of this fundamental frequency. The fundamental is the first resonance and is also known as the first harmonic, whereas the subsequent resonances are called overtones and harmonics. It's important to note that the first overtone is the second harmonic, and so on, with each overtone corresponding to a higher number harmonic.