The metre is the fundamental unit of length in the International System of Units (SI). It is defined as the distance light travels in a vacuum in 1/299,792,458 seconds. This precise definition allows for consistent measurements across various scientific and practical applications. Originally, the metre was based on the Earth's meridian, specifically one ten-millionth of the distance from the North Pole to the Equator. Today, it is widely used in many fields, including science, engineering, and everyday life, making it a crucial part of the global measurement system.