What defines a longitudinal wave?

A longitudinal wave is characterized by the movement of particles in the same direction as the wave's propagation. This means that as the wave travels, the particles of the medium oscillate back and forth along the same line as the wave moves forward. This creates regions of compression (where particles are closer together) and rarefaction (where particles are more spread out) along the wave.

In the context of the other statements, the assertion about perpendicular movement describes transverse waves, where particles move at right angles to the direction of wave propagation. The claim that longitudinal waves can only travel through solids is not accurate, as they can also propagate through liquids and gases. Lastly, the idea that all particles move uniformly does not apply to longitudinal waves because the particles exhibit oscillatory motion rather than moving in sync. Thus, the defining characteristic of longitudinal waves is indeed the parallel movement of particles to the direction of the wave.