A system of natural units is a system in which certain fundamental constants are chosen to be equal to unity. Omitting them from equations simplifies the notation and clarifies the underlying physics.

In particle physics, the standard choice is to set c = ℏ = 1, such that c = 1 removes the distinction between space and time, while ℏ = 1 removes the distinction between energy and frequency (or inverse time). The electron‑volt (eV) and its multiples, MeV (10⁶ eV), GeV (10⁹ eV), and TeV (10¹² eV), originally units of energy, can be used to express all quantities with dimensions of length, time, or mass in terms of powers of energy. As a consequence, all explicit factors of c and ℏ disappear, simplifying the notation and conversions.