Volumetric strain is the ratio between change in volume due to applied load and original volume. Its SI unit is dimensionless.

Strain is the ratio between change in length due to applied load and original length. Its SI unit is dimensionless.

Critical Mach number is the lowest Mach number at which the airflow over the aircraft reaches Mach number 1.

Downstream Mach number is after shock speed of fluid particle unknown as downstream Mach number.

Modulus of rigidity is the ratio of shear stress to shear strain. Its SI unit is N/m^2.