What does the term longitudinal wave mean when applied to sound?

Answer 1

It simply describes the type of wave phenomenon it is, and sound is a longitudinal wave because the motion energy that propagates the wave at a microscopic scale has to do with molecules vibrating along the axis of the propagation.

The other type of mechanical wave would be a transverse wave, such as with a guitar string. The energy is added to the string by plucking it in the transverse direction, and that transverse motion propagates at right angles to the direction of propagation. Ocean waves are similar.