The electron cyclotron frequency refers to an electron's orbit in a magnetic field. The frequency of this orbit is given as eB/m where e is elementary charge, B is magnetic field strength and m is mass of electron. The radius of this orbit (either called the cyclotron radius or the Larmor radius) depends on the kinetic energy of the electron perpendicular to the magnetic field and is given by m*u_perp/(eB) where u_perp is the velocity of an electron perpendicular to the magnetic field.
Why it is called the "cyclotron" I am not sure. However, it is always called cyclotron when referring to this kind of motion, which is typically the smallest orbit and the highest frequency, as opposed to other "guiding center" orbits (e.g. magnetron) an electron can have in magnetic confinement devices.