# rapidity

> English word · Noun · IPA /ɹəˈpɪd.ɪ.ti/ · frequency rank #61,748

## Definitions
1. speed, swiftness; the condition of being rapid
2. A measure of velocity relative to the speed of light; defined as artanh(v), where artanh is the hyperbolic arctangent, and v = speed (with c = speed of light = 1).
3. A measure of the velocity of a particle in a beam relative to the beam's axis

## Etymology
From rapid + -ity, from French rapidité, from Latin rapiditas.

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/rapidity