Final answer:
In a longitudinal wave, the areas where the medium's particles are crowded together are called compressions.
Step-by-step explanation:
The part of a longitudinal wave where particles of the medium are crowded together is known as compression. In longitudinal waves, particles of the medium move in a direction parallel to the wave propagation. These waves consist of areas where the particles are close together (compressions) and areas where they are spread apart (rarefactions). The wavelength of a longitudinal wave is the distance between two consecutive compressions or two consecutive rarefactions.