The gyrofrequency, also known as the cyclotron frequency or Larmor frequency, is the frequency of rotation of a charged particle or ion as it spirals a magnetic field.

The trajectory of a charged particle acted on solely by a magnetic force is a helix with its axis parallel to the magnetic induction. In any plane perpendicular to the magnetic induction the particle describes a circular orbit with gyrofrequency qB/m, where q is charge, m is mass, and B is the magnitude of magnetic induction.