Wavelength defination

Wavelength is equal to the distance travelled by the wave during the time in which any one particle of the medium completes one vibration about its mean position. or

Wavelength is the distance between any two nearest particles of the medium, vibrating in the same phase. or

Distance travelled by the wave in one time period is known as wavelength. or

It is the length of one wave.

Wavelength is denoted by λ

Wave or the wave motion are two types that is Transverse wave motion or Longitudinal wave motion, wavelength is different for these two type of wave motion i.e.

In transverse wave motion :

λ = Distance between the centres of two consecutive crests.
λ = Distance between the centres of two consecutive troughs.
λ = Distance in which one trough and one crest are contained.

In longitudinal wave motion :

λ = Distance between the centres of two consecutive compression.
λ = Distance between the centres of two consecutive rarefaction.
λ = Distance in which one compression and one rarefaction contained.