${\displaystyle \beta :=\frac{v}{c_0}}$, and $\beta =\left|\overrightarrow{\beta }\right|$

Note 1 to entry: The normalized speed for transfer of energy or information is less than or equal to 1. For transfer of matter, it is always less than 1.

Note 2 to entry: The coherent SI unit of normalized velocity is one, symbol 1.