The pressure coefficient is a dimensionless quantity of interest to aerodynamicists.

The pressure coefficient is defined as:

p - p_inf = q_inf . C_p

where:
C_p is the dimensionless pressure coefficient
p is the pressure due to the movement of the body through the fluid
p_inf is the free stream fluid pressure
p - p_inf is known as the gauge pressure
q_inf is the free stream fluid dynamic pressure from Bernoulli's equation:

q_inf = 1/2 rho_inf . V_inf²

where:
rho_inf is the free stream fluid density
V_inf is the free stream fluid speed

Pressure Coefficient Comparison Between Calculation (lines) and Experiment Around an Airfoil

You will often see pressure coefficient plots of numerical calculations compared with wind tunnel experimental data as a means to judge the accuracy of the numerical technique used. This type of comparison is often referred to as a validation.