Definition of natural unit in US English:

natural unit

noun

  • 1A unit of measurement that is considered to be natural; especially one based on the fundamental properties of the object, space, system, etc., under consideration.

  • 2Physics. A unit of measurement in a system in which certain fundamental constants (the speed of light c, Planck's constant h or ℏ, and the constant of gravitation G) are set equal to unity.

Origin

Early 19th century; earliest use found in The North American Review. From natural + unit [adjective].