Answer : Wavelength.
Explanation :
A disturbance created in a medium produces wave.
In transverse wave, the particles of the medium moves in the direction perpendicular to the direction of propagation of wave.
The uppermost portion of the wave is called the crest and the lower most portion is called the trough.
In longitudinal wave, the particles of medium moves in the same direction of wave i.e. in the direction parallel to the wave.
The particles of medium in longitudinal wave forms compressions and rarefactions. In compression the density of particles in more in a particular space while in rarefaction it is less.
Hence, the wavelength is the the distance between two crests or two troughs on a transverse wave.
It is also the distance between compressions or the distance between rarefactions on a longitudinal wave.
