Extent of something along its greatest dimension or the extent of space between two objects or places. The metric system unit for this quantity is "m" (metre). The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s⁻¹, where the second is defined in terms of the caesium frequency ∆νCs. This definition implies the exact relation c = 299 792 458 m s⁻¹. Inverting this relation gives an exact expression for the metre in terms of the defining constants c and ∆νCs: 1 m = (c / 299 792 458)s = 9 192 631 770 c / 299 792 458 ∆νCs ≈ 30.663 319 c / ∆νCs

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SI Base Unit