ECEF stands for Earth-Centered, Earth-Fixed, and is a Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system[1]. 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[2], 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[3][4], hence the name Earth-Fixed.
The z-axis is pointing towards the north but it does not coincide exactly with the instantaneous Earth rotational axis[3]. The slight "wobbling" of the rotational axis is known as polar motion[5]. The x-axis intersects the sphere of the earth at 0° latitude (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.