Reduced Planck constant (Dirac constant)

Dirac's h-bar is just h/2π, but the 2π is not cosmetic: it converts "per cycle" into "per radian". Anywhere a phase, an angular frequency ω or an angular momentum appears, ħ is the constant that belongs there — E = ħω, p = ħk, [x, p] = iħ, and the Schrödinger equation itself. Electron spin is ħ/2; orbital angular momentum comes in whole multiples of ħ.

Because h and π are both exact, ħ is exact too — but its decimal expansion never terminates, so tables print a truncation. The value here is the standard 10-digit form; use h/(2π) directly if you need more. The classic error is mixing conventions inside one calculation: E = hf and E = ħω give the same answer only if you also convert f to ω = 2πf, and a stray factor of 6.28 in a quantum-mechanics problem is almost always this.