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The definition of the meter has evolved over time. Four key definitions, in chronological order, are particularly noteworthy: **Pendulum meter** This definition was based on the length of a pendulum, where the duration of one half-cycle (a single swing, or half an oscillation) takes exactly one second. **Meridional meter** The meridional meter was defined as one ten-millionth of the distance between the North Pole and the equator along the Paris meridian. This established the Earth's circumference as exactly 40,000 kilometers. **Prototype meter bar** The prototype meter bar was an international standard defined as the distance between two marks on a bar made of an alloy containing 90% platinum and 10% iridium, measured at the temperature of the melting point of ice. **Wave meter** The current definition of a meter is the distance light travels in a vacuum during the time interval of 1/299,792,458 of a second.
