What is pressure? 

Pressure is basically Force / Area and is the magnitude of an external force applied to a unit area. A scalar measure of pressure has only magnitude in it. Pressure is basically equal at all points and in all directions. If there is pressure on a surface, it always acts perpendicular to the surface. And this is considered a special case of stress. It is always compressed and is used for fluids, liquids and gases. It is usually specified in N/m^2. And it is commonly measured by various measuring devices like manometer, bourdon tube, diaphragm etc.