What is the difference between speed and velocity?
Velocity
Speed describes only how fast an object is moving, whereas velocity gives both how fast it is and in which direction the object is moving.[1] If a car is said to travel at 60km/h, its speed has been specified. However, if the car is said to move at 60km/h to the north, its velocity has now been specified.
Propagation of light in non-inertial reference frames
The description of motion in relativity requires more than one concept of speed. Coordinate speed is the coordinate distance measured by the observer divided by the coordinate time of the observer. Proper speed is the local proper distance divided by the local proper time. For example, at the event horizon of a black hole the coordinate speed of light is zero, while the proper speed is c. The coordinate speed of light (both instantaneous and average) is slowed in the presence of gravitational fields. The local instantaneous proper speed of light is always c.