Maximum usable frequency

Maximum usable frequency (MUF) describes, in radio transmission, using reflection from the regular ionized layers of the ionosphere, the upper frequency limit that can be used for transmission between two points at a specified time. This index is especially useful in regards to shortwave transmissions.

MUF is a median frequency applicable to 50% of the days of a month, as opposed to 90% cited for the lowest usable high frequency (LUF) and the Frequency of optimum transmission (FOT).

Original source: From Federal Standard 1037C and from MIL-STD-188