Terminology and jargon used throughout Cesium Native and its documentation is collected here for clarity's sake.

Earth-Centered, Earth-Fixed Coordinates (ECEF)

Earth-Centered, Earth-Fixed (ECEF) coordinates are, as the name describes, in a 3D Cartesian coordinate system fixed to the Earth with the center at the center of the Earth's ellipsoid. As the Earth spins, the coordinate system spins with it, meaning an ECEF coordinate and its equivalent coordinate in a cartographic coordinate system (like Longitude, Latitude, Height) will remain the same.

For example, Philadelphia, Pennsylvania is located at -75.1652215° longitude, 39.952839° latitude, at a height of 14.34m. The equivalent ECEF coordinates are (1253556.69, -4732887.41, 4073982.02).