What is Wavelength?

In short

Wavelength is the distance between two neighbouring points of a wave that are in the same state, for example from one crest to the next crest. It is measured in metres.

In simple words

It is written with the Greek letter $\lambda$ (lambda). Sound in air, light, waves on a string and waves on water all have a wavelength. For a wave of fixed speed, a longer wavelength means a lower frequency. The visible light we see has wavelengths of roughly $400$ to $700\ \text{nm}$ .

Example

A wave on a rope has crests that are $0.50\ \text{m}$ apart, so its wavelength is $\lambda = 0.50\ \text{m}$ . If $4$ crests pass a point every second, the wave speed is $0.50\ \text{m} \times 4\ \text{s}^{-1} = 2.0\ \text{m/s}$ .