A stationary wave is set up in a bounded medium in which the boundary could be a rigid support (i.e., a fixed end, as for instance a string stretched between two rigid supports) or a free end (as for instance an air column in a cylindrical tube with one or both ends open). The boundary conditions limit the possible stationary waves and only a discrete set of frequencies is allowed.
The lowest allowed frequency n1 is called the fundamental frequency of vibration. Integral multiples of the fundamental frequency are called the harmonics, the fundamental frequency being called the first harmonic. The second harmonic is twice the fundamental or 2n1, the third harmonic is 3n1, and so on.
The higher allowed harmonics above the first harmonic or fundamental are called overtones. The first overtone is the higher allowed harmonic immediately above the first harmonic.
Above the fundamental, the first allowed frequency is called the first overtone, the next higher frequency is the second over-tone, and so on. The relation between overtones and allowed harmonics depends on the system under consideration.
