What does MUF stand for?

MUF stands for Maximum Usable Frequency. Like the LUF, it is entirely dependent on the specific path between two points on Earth at any given moment. If a transmission uses a frequency higher than the MUF for a given path, the radio wave will pass straight through the ionosphere into space instead of being refracted back down to the receiving station. It is crucial to remember that 'M' stands for Maximum, not Minimum. Also, the MUF is a real-time, path-specific metric, so answers suggesting it is a peak over a 24-hour period are incorrect.