A monitor unit (MU) is a measure of machine output of a linear accelerator in radiation therapy.
Linear accelerators are calibrated to give a particular absorbed dose under particular conditions. One standard calibration is such that 100 MU gives an absorbed dose of 1 Gray (or 100 rad) at a depth of 5 cm in tissue-equivalent phantom, for a 10x10 cm2 field at 95 cm source-to-surface distance (SSD).
Some linear accelerators are calibrated using source-to-axis distance (SAD) instead of SSD, and calibration (monitor unit definition) may vary depending on hospital custom.