From reading the articles before this one, you may have noticed that pressure also plays a role in thermodynamics in addition to temperature. In fact a change in temperature could have an effect on the pressure of the system being studied, depending on how you are defining the process when the change takes place.
So what is a pressure? Simply, in thermodynamics as well as fluid mechanics pressure is defined as a normal force exerted by a fluid per unit area. The reason why it has to be a normal force is because if a fluid, whether it is gas or liquid, has a shear stress of any magnitude applied to it the fluid particles will flow over each other and will not exert any resistive force back; assuming that the fluid in inviscous.
A gas and a liquid will however behave differently in relation to each other when under pressure. The above statement still holds true that a liquid or gas will exert a normal force per unit area in respect to the pressure on the liquid or gas. However, the difference is a liquid is normally considered incompressible, while a gas is normally considered compressible. What this means is when the pressure increases on a liquid the volume can be assumed to remain the same as well as the density of the liquid, unless of course it is subject to extremely high pressure changed. Gas on the other hand will compress, meaning that as the pressure increases the volume of the gas will decrease while the density increases, if the gas is within a closed system. The opposite will occur if the pressure decreases on a gas.
Now when you are dealing with pressure you need to know if your pressure is in absolute pressure or if it is gage pressure. Absolute pressure means that both the atmospheric pressure and the gage pressure are being considered. Typically the atmospheric pressure on earth at sea level is 101.325 kPa or 14.7 psi. Gage pressure on the other hand does not consider atmospheric pressure. In addition to absolute pressure and gage pressure you may hear people talking about a vacuum pressure, which means the pressure is below atmospheric pressure.
Most instruments that measure pressure measure gage pressure. Some of these instruments that measure gage pressure are as follows: Bourdon tubes, pressure transducers, strain gages, and manometers.
If you want to learn more about how to calculate pressure refer to the Fluid Mechanics section of this website. There you will learn how to calculate pressure at different depth, as well as how pressure changes when fluid velocity changes.