Understanding the difference between gauge and absolute pressure sensing measurements is easiest to visualize with piezoresistive type pressure sensor and a diaphragm that deflects with pressure. For both types of measurements, pressure is applied to the top of the thin diaphragm. With the gauge pressure sensor, the back side is vented to atmosphere. This could be called a basic pressure measurement.

Modified  version of All Sensors graphic.

Absolute pressure measurements are made with respect to a fixed pressure reference sealed within the sensor. Normally, the reference is a vacuum, so the diaphragm is pulled toward the vacuum, the center of the sensor, and is deflected in its resting state. With the sealed (vacuum) reference, the absolute pressure sensor can be used for barometric, altitude and underwater measurements.

There are special cases of gauge measurements: gauge vacuum and differential pressure.

Gauge vacuum measurements are made by applying vacuum to the back side of the diaphragm of a gauge sensor, where it would normally vent to the atmospheric pressure. In this case, the top side of the sensor is exposed to the atmosphere.

With differential pressure, the back side of the gauge type sensor is connected to a low-pressure source and the top side is connected to a high-pressure source. The measurement is the difference between the two pressures.

In many cases, it is important to identify the type of pressure to avoid confusion and error. However, there are also many cases where the type of measurement is understood. For example, tire pressure (gauge), barometric pressure (absolute) and flow pressure (differential).