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 extends through True north, which does not coincide 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 (prime meridian in 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.