The difference between the orthometric and ellipsoidal height at a point is known as:

The difference between the orthometric height (which is the height above the geoid) and the ellipsoidal height (which is the height above the reference ellipsoid) is referred to as geoidal height. This measurement is crucial in geodesy, as it enables the conversion between ellipsoidal heights, obtained from GPS measurements, and orthometric heights, which are used in traditional leveling methods.

Geoidal height accounts for the irregularities in the Earth's shape and gravitational field, which are not represented by a simple ellipsoid. By understanding the geoidal height, geodetic engineers can accurately relate the measurements taken from different reference surfaces, which is essential for applications such as surveying, mapping, and navigation.

The other options do not accurately define the difference between orthometric and ellipsoidal heights. The height of the geoid refers more to the general concept of the model of the Earth's surface, vertical datum pertains to a specific reference point or surface used for elevation measurements, and hydrostatic pressure relates to fluid mechanics rather than geodetic heights.