Final answer:
The meter is defined by the International System of Units (SI) as the distance light travels in 1/299,792,458 of a second. The metric system is not completely adopted by three countries: Liberia, Myanmar, and the United States, the latter using the system legally but not in everyday practice.
Step-by-step explanation:
The meter, which is a fundamental unit of length in the metric system, was originally based on one ten-millionth of the distance from the equator to the North Pole along a meridian passing through Paris. However, since 1983, the meter has been officially defined by the International System of Units (SI) as the length of the path travelled by light in a vacuum in 1/299,792,458 of a second. This definition provides a universal standard that can be precisely reproduced in laboratories around the world.
While the metric system is widely adopted by most countries, only three countries in the world do not use it for making measurements: Liberia, Myanmar, and the United States. However, it's important to note that while the U.S. has legally adopted the metric system, it is not commonly used in everyday practice. Instead, the U.S. continues to use primarily imperial units for most applications.