Question: why the sound waves are called longitudinal waves

title: Longitudinal wave
text: y ( x , t ) = y 0 cos ⁡ ( ω ( t − x c ) ) ( \ displaystyle y ( x , t ) = y_ ( 0 ) \ cos ( \ Bigg ( ) \ omega \ left ( t - ( \ frac ( x ) ( c ) ) \ right ) ( \ Bigg ) ) )