What is the Planck constant divided by 2π called?

Planck’s constant divided by 2π is called the reduced Planck constant, written ħ or “h-bar.”

The relationship is ħ = h/(2π), where h is the Planck constant. The reduced form appears naturally whenever physics uses angular frequency or angular momentum. For example, photon energy can be written E = ħω using angular frequency ω, instead of E = hf using ordinary frequency f.

Max Planck introduced h in 1900 while studying black-body radiation, proposing that energy exchange occurs in discrete quanta. The constant later became central to quantum mechanics. The factor 2π enters because a complete cycle of oscillation corresponds to 2π radians, so angular descriptions make ħ more convenient than h.

The reduced Planck constant is not an electronvolt, which is a unit of energy, nor is it simply another name for h. Its approximate value is 1.054571817 × 10⁻³⁴ joule-seconds. It also appears in the uncertainty principle, Schrödinger’s equation and quantum angular-momentum rules such as L = √(ℓ(ℓ+1))ħ.