Absolute pressure is defined as:

Absolute pressure is defined as the total pressure exerted on an object, including atmospheric pressure, measured from a perfect vacuum. In this context, "zero-referenced against a vacuum" means that absolute pressure accounts for all pressures acting on a system, eliminating any variations caused by atmospheric conditions. This makes it an essential concept in fields such as physics and engineering, allowing for more precise calculations related to gas laws and fluid dynamics.

By understanding that absolute pressure includes the weight of the atmosphere above the measurement point plus any additional pressure from within a system, one can assess conditions accurately whether above or below the surface of a liquid. This distinction is critical in scuba diving, as diver depth and buoyancy calculations depend significantly on absolute pressure to avoid risks such as decompression sickness or equipment failure.

In contrast, pressure above sea level does not provide a complete picture, as it only accounts for pressure differences from a designated reference point rather than from a vacuum, while pressure relative to atmospheric conditions does not encompass the full atmosphere pressure. Similarly, limiting the definition to underwater pressure ignores the importance of atmospheric pressure, which is vital for accurate pressure measurements in relation to diving safety and equipment performance.