The wavelength is a property of a waveform, a famous example of a waveform being a sinusoid. As I am sure you are aware, it is simply the distance from crest to crest. Waveforms can exist in time, as in the pendulum example; and waveforms can exist in space as in a snapshot of the ocean. The wavelength of a pendulum is simply the time it takes to complete one oscillation.

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