• in classical mechanics, by $T=\frac{1}{2}m{v}^2$, where $m$ is mass and $v$ is speed;
• in relativity theory, by $T=m_0{c}_0^2(\gamma -1)$, where $m_0$ is rest mass and $\gamma$ is the Lorentz factor

Note 1 to entry: In classical mechanics, the kinetic energy of a body is given by the integral $T=\frac{1}{2}{\displaystyle {\int }_{\text{D}}{v}^2}\text{d}m$, where $\mathrm{D}$ is a domain containing the body. When the axis of rotation is through the centre of mass, $T$ is also given by $T=\frac{1}{2}{m}_{\text{t}}{v}_{\text{G}}^2+\frac{1}{2}J{\omega }^2$, where $m_{\text{t}}$ is the total mass, $v_{\text{G}}$ is the speed of the centre of mass, $J$ is the moment of inertia relative to the axis of rotation, and $\omega$ is the magnitude of the angular velocity, and where $v_{\text{G}}$, $J$, and $\omega$ can be time-dependent.

Note 2 to entry: For low velocities in relativity theory, $T=m_0{c}_0^2(1+\frac{1}{2}{\beta }^2+\frac{3}{8}{\beta }^4+\cdots -1)\approx \frac{1}{2}{m}_0{v}^2$, where $\beta$ is the normalized speed.

Note 3 to entry: The coherent SI unit of relativistic kinetic energy is joule, J.