From AMS Glossary
(Also cyclotron frequency, Larmor frequency.) The frequency of a moving charged particle or ion in 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 gyro-frequency qB/m, where q is charge, m is mass, and B is the magnitude of magnetic induction.