A longitudinal wave is a mechanical wave. (meaning it requires a medium to travel.) These waves consist of “compressions,” and “rarefactions.” Compressions are areas where the where the waves are closer together, rarefactions are where they are farther apart. The illustration below allows you to visualize it.
Source: http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/sound/tralon.html
Some examples of longitudinal waves include: Sound Waves and Radio Waves.
Project Wave 
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