$\text{d}{s}^2={\displaystyle \sum _{\mu ,\nu }{g}_{\mu \nu }}\text{d}{x}^{\mu }\text{d}{x}^{\nu }$

where $g_{\mu \nu }$ is a component of the metric tensor and $\text{d}{x}^{\text{μ}}$, $\text{d}{x}^{\text{ν}}$ are components of a quasi-infinitesimal position four-vector

Note 1 to entry: From the mathematical point of view, the squared interval is rather a “squared length of interval” or “squared space-time range” rather than a “squared interval” or even an “interval”. These two terms are also used in literature.

Note 2 to entry: The coherent SI unit of squared interval is metre squared, ${\text{m}}^2$.