Figure 9.11 - Compression Wave with expanded and contracted Orbits

Same as Longitudinal Wave or Sound Wave. A wave incident or normal to a surface. One of many modes or dimensions of motion present in all vibrations and oscillations. Generally, considered as a motion to and from a center or source. When harmonic is considered syntropic and when enharmonic considered entropic.
"Vibration in which the principal motion is in the direction of the longest dimension." (Rossing, Thomas D.; The Science of Sound; Addison-Wesley Publishing Company, 1982.)
"A vibrating medium must lengthen and shorten 2 times per each lateral (transverse) oscillation; so 1 longitudinal wave equals 2 lateral or (transverse). Longitudinal wave frequencies are independent of string tension." (Tyndall, John; Sound; Longmans, Green, and Co., London, 1893.)

Figure 9.11 - Compression Wave with expanded (enharmonic) and contracted (harmonic) orbits