The distance from the peak to a peak of a successive wave.

Wavelength

Explanation:

Wavelength is the horizontal distance between the peaks of successive troughs or crests of a wave. Wavelength is measured in meters and when you have the wavelength you can also calculate the frequency when velocity of the waves are known since velocity of wave is given as the product of wavelength and frequency. Conclusively, the distance as described from peak to peak of successive wave is termed as wavelength