In radio transmission, maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points on Earth by reflection from the ionosphere (skywave or skip) at a specified time, independent of transmitter power.
In shortwave radio communication, a major mode of long distance propagation is for the radio waves to reflect off the ionized layers of the atmosphere and return diagonally back to Earth.
Above this frequency the radio waves are not reflected by the ionosphere but are transmitted through it into space.
Given the maximum observed frequency (MOF) for a mode on each day of the month at a given hour, the MUF is the highest frequency for which an ionospheric communications path is predicted on 50% of the days of the month.
Commonly, the optimal operating frequency for a given path is estimated at 80 to 90% of the MUF.
As a rule of thumb the MUF is approximately 3 times the critical frequency.