A series of systems usually derived from the International System of Units that are more convenient for certain applications.
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Natural units choose units so that selected fundamental constants have numerical value one. In relativistic quantum field theory the standard choice is : energy, mass and inverse length then have the same units. This is a convention, not a physical change of the constants; other natural-unit systems may also normalize other constants.
Natural units are a system of units in physics in which certain physical constants are set to 1. This simplifies many equations and relationships by eliminating the need for explicit reference to these constants. The most commonly used natural units involve the speed of light (\(c\)), the gravitational constant (\(G\)), and the reduced Planck constant (\(\hbar\)).