ECEF ("earth-centered, earth-fixed"), also known as ECR ("earth-centered rotational"), is a geographic coordinate system and Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system. It represents positions as an X, Y, and Z coordinate. The point (0,0,0) is defined as the center of mass of the earth, hence the name "earth-centered." Its axes are aligned with the international reference pole (IRP) and international reference meridian (IRM) that are fixed with respect to the surface of the earth, hence the description "earth-fixed." This term can cause confusion since the earth does not rotate about the z-axis (unlike an inertial system such as ECI), and is therefore alternatively called ECR.
The z-axis is pointing towards the north but it does not coincide exactly with the instantaneous earth rotational axis. The slight "wobbling" of the rotational axis is known as polar motion. The x-axis intersects the sphere of the earth at 0° latitude ( the equator) and 0° longitude ( Greenwich). This means that ECEF rotates with the earth, and therefore coordinates of a point fixed on the surface of the earth do not change. Conversion from a WGS84 datum to ECEF can be used as an intermediate step in converting velocities to the north east down coordinate system.