What is "four-current"

In special and general relativity, the four-current (technically the four-current density) is the four-dimensional analogue of the electric current density, which is used in the geometric context of four-dimensional spacetime, rather than three-dimensional space and time separately. Mathematically it is a four-vector, and is Lorentz covariant.

Analogously, it is possible to have any form of "current density", meaning the flow of a quantity per unit time per unit area, see current density for more on this quantity.

This article uses the summation convention for indices, see covariance and contravariance of vectors for background on raised and lowered indices, and raising and lowering indices on how to switch between them.