Complex sentence: In physics, in particular in special relativity and general relativity, a four-velocity is a four-vector in four-dimensional spacetime that represents the relativistic counterpart of velocity, which is a three-dimensional vector in space.
Split into simpler sentences: In physics, in particular in special relativity and general relativity, a four-velocity is a four-vector in four-dimensional spacetime. It is the relativistic counterpart of velocity, which is a three-dimensional vector in space.