${\displaystyle Grad\rho :=\left(\frac{\partial \rho }{\partial x_0},\frac{\partial \rho }{\partial x_1},\frac{\partial \rho }{\partial x_2},\frac{\partial \rho }{\partial x_3}\right)=\left(\frac{\partial \rho }{\partial \text{j}{c}_{0}t};grad\rho \right)}$ where $\rho$ is an arbitrary scalar field quantity

Note 1 to entry: Four-gradient is useful in STR. In general theory of relativity (GTR) for non-flat space-time, a more sophisticated method is used.

Note 2 to entry: Four-gradient of a tensor quantity $Q$ of order $n\ge 0$ is a direct product of four-nabla and quantity $Q$: $GradQ=◊Q$ yielding a tensor quantity of order $n+1$.

Note 3 to entry: In the International System of Quantities, the dimension of four-gradient is ${\mathrm{L}}^{-1}$.