four-velocity
Technical term in relativity; the four-vector analogue of velocity in Minkowski spacetime.
In special relativity, the magnitude of the four-velocity is always equal to the speed of light.
General term for a four-vector; 四元速度 is a specific instance of a four-vector representing velocity.
Ordinary three-dimensional velocity; 四元速度 extends this concept to four-dimensional spacetime.
Compound of 四元 (four-dimensional) and 速度 (velocity), directly translating the English physics term 'four-velocity'.