The standard unit of length in the metric system is based on what.
The standard unit of length in the metric system is based on the meter.
The standard unit of length in the metric system is based on the distance traveled by light in a vacuum in a specified amount of time. This unit is called the meter (m).
To understand how the meter is defined, we need to delve into the science of physics. The speed of light in a vacuum is a fundamental constant of nature, denoted by the symbol "c." It is approximately equal to 299,792,458 meters per second (m/s). The meter is defined as the distance that light travels in a vacuum in 1/299,792,458th of a second.
Since the speed of light is a physical constant, the meter is considered an absolute and reproducible unit of length. It provides a consistent and standardized measurement system for scientific and everyday purposes within the metric system.
This site gives the current definition but also the history before.
http://dictionary.reference.com/browse/meter