What is the height of the sea's surface generally called?

The height of the sea's surface is generally called sea level, more precisely mean sea level when it refers to an averaged reference surface. Because waves, tides, winds, currents, and air pressure constantly move the ocean, there is no single perfectly fixed water height at every moment.

Surveyors and mapmakers use mean sea level as a vertical reference for describing elevation. A mountain's height, a valley's depth, and an airport's altitude may all be reported as distances above or below this reference. Local mean sea level is calculated from long-term observations at a particular coastal location, often using tide gauges.

Sea level is also important in climate science. Global sea level changes as ocean water warms and expands, as land-based ice melts, and as the shape or volume of ocean basins changes. Local measurements can differ because land itself may rise or sink. GPS heights can also differ from sea-level-based elevations because GPS commonly references a mathematical model of Earth's shape rather than the local ocean surface.