Pressure (symbolized by P) is the effect of a normal force acting on an area1. Mathematically pressure is force/area. For an ideal gas pressure can be estimated by (density*R*T), where R is the universal gas constant and T is the temperature in Kelvin. Atmospheric pressure (1 atm) is the pressure which is exerted by the weight of the atmosphere at sea level, and is equivalent to 101.325 kPa (or 2.28 lbf in English units). Atmospheric pressure changes in relation to vertical distance from the ocean. For instance, in Denver, the "mile high city", atmospheric pressure is only 84 kPa.
As pressure increases, water has a decreasing volume at the point where it turns into a gas.
A gauge is a device that reads pressure. A negative gauge pressure implies that what is being measured is a vacuum. A gauge that can read negative pressures is a vacuum gauge. Absolute pressure can be found as the sum of gauge pressure with atmospheric pressure.