Reverse leakage
also known as: I_R · Leakage current
The small current that flows backwards through a diode that is supposedly blocking. An OFF diode isn't a wall — datasheets specify I_R at a given reverse voltage, from nanoamps (small-signal silicon) to milliamps (large Schottkys, hot).
In practice
It matters most on batteries, where the leak runs 24/7: a few microamps of 'protection' diode can out-drain the microcontroller it protects. Leakage also rises steeply with temperature — the datasheet number at 25 °C can be 100× worse at 85 °C.