What is the difference between gauge and absolute pressure called?

The difference between gauge pressure and absolute pressure reflects a fundamental concept in fluid mechanics and pressure measurement. Gauge pressure measures pressure relative to the ambient atmospheric pressure, meaning it disregards the atmospheric pressure acting on a fluid. On the other hand, absolute pressure measures the total pressure including atmospheric pressure acting on the fluid.

The term that denotes this difference is "Relative pressure," which specifically refers to pressure measurements that account for variations in atmospheric pressure. The atmosphere, or atmospheric pressure, itself is not a measure of the difference between gauge and absolute pressure, and nor is "atmospheres absolute," which can often be associated with the standard atmosphere value.

"Manometric pressure" often relates specifically to the pressure indicated by a manometer and can be synonymous with gauge pressure in certain contexts, but it does not precisely define the difference between gauge and absolute pressure as understood in general pressure terminology.

Hence, the correct term to describe the difference between gauge and absolute pressure is indeed "Relative pressure," as it emphasizes the relationship and how gauge pressure is a subset of pressure measurements that are relative to the surrounding atmospheric conditions.