Hello Siti

In term of history, the pressure coefficient comes from aerodynamics : when an uniform flow is coming on a structure, you can define a "dynamic pressure", calculating half density times square of flow speed: 1/2 rho U² (homogeneous to a pressure). Bernoulli's equation states that the in this kind of flow, the "total pressure" which is the sum of actual pressure and dynamic pressure, is constant (with a lot of hypothesis: laminar and constant flow, incompressible, no height potential...). So p + 1/2 rho U² is constant.

There is a concept of pressure coefficient: Cp=(p-Pref)/(1/2.rho.Uo²). Generally , this coefficient is less than 1 ; =1 on the upstream face of the object (stagnation point), decreases to 0 when the flow is parallel to the surface, and is negative in the wake of the object. There is no negative limit, but a coefficient exceeding -3 is quite rare.

Best regards.