Pressure, from the latin pressĭo, is a term that refers to the action and the effect of the compress or squeeze. Can be oppression applied on something or someone, the physical quantity measured in Pascals indicating the force exerted by a body on a unit area.
Absolute, in turn, is an adjective that relates to what is unlimited, independent, full or total. The absolute is unconditional (there by itself, without necessary relationship).
Absolute pressure means any real pressure on a given point. The concept relates to atmospheric pressure and the gauge pressure.
Atmospheric pressure is the weight exerted by the air at a given point of the atmosphere (the layer of gas that is around the planet). This pressure on the Earth varies according to the altitude: more altitude is important, less atmospheric pressure is important.
The pressure gauge, on the other hand, is one that produces a different way of atmospheric pressure (for example, that exerted by the gas of a drink or a soda on the bottle).
These two concepts allow us to return to the idea of absolute pressure, which is calculated in a given area from the sum of atmospheric pressure with the gauge pressure (gauge pressure). Taking, for example, a bottle of Coca-Cola, the absolute pressure to which it is subject is equal to the sum of atmospheric pressure (outside packaging) and the pressure (internal, by the action of the molecules of the gas of the drink).