In Longitudinal waves, particles of the medium vibrate around their mean positions. Their amplitude of vibration is in the direction of the propagation of the wave. In transverse wave of longitudinal wave, the wavelength is always the distance between two particles which are in the same phase.
If we take pressure waves, (sound waves), we have pressure variations created by sound wave along its path. Pressure is maximum at compression regions and pressure is minimum at rarefaction region. In between the two, pressure of air remains as the pressure when there is no wave.
The wave length is then the distance between two consecutive rarefactions or two consecutive compression regions.
It is also the distance traveled by the wave in one time period. Time period is the time the particles in the medium take to vibrate towards the end, turn back to reach the other end of their oscillation and then reach back their position.
