I'm currently reading/studying the FLP and I have a question regarding waves. In a chapter about QM, Feynman says that any short wave train doesn't have a definite wavelength. I understand this, because the wave train is composed of a number of frequencies of sine waves (revealed by a Fourier transform). The only [sinusoidal] waves that have a definite wavelength are infinite perfect sines (disregarding any phase difference).
My question is: what happens if the waveform is not a sinusoid? If the waveform is a [infinite] square wave, for example, it can be analyzed via Fourier to reveal that it's composed of several sinusoids of different wavelengths, but on the other hand, one can actually look at the pattern and measure a "definite wavelength".
In the image above, the wavelength I'm speaking about is $v * 10ms$, where v is the velocity of the wave [For example, assuming 1000 m/s, then $\lambda = 10 meters$]. Does make sense to speak of a definite wavelength in this case?