Note 1 to entry: Four-acceleration is a four-vector analogous to acceleration in three-dimensional space.

Note 2 to entry: $\underset{\_}{\underset{\_}{a}}:=\text{d}\underset{\_}{\underset{\_}{u}}/\text{d}\tau =\left(\text{j}\gamma \dot{\gamma }{c}_0;\gamma \dot{\gamma }\overrightarrow{u}+{\gamma }^2\dot{\overrightarrow{u}}\right)$ where $\gamma$ is the Lorentz factor and $\dot{\gamma }$ its derivative, $\text{j}$ is the imaginary unit, and $c_0$ is speed of light in vacuum.

Note 3 to entry: As the four-scalar product $\underset{\_}{\underset{\_}{a}}\cdot \underset{\_}{\underset{\_}{u}}=0$, the four-acceleration is orthogonal to four-velocity.

Note 4 to entry: The coherent SI unit of four-acceleration is metre per second squared, $\text{m}\cdot {\text{s}}^{-2}$.