Dark Frame
An exposure made with the shutter closed, at the same duration, temperature and ISO as the real image, used to characterise sensor noise.
In depth
Because dark current, hot pixels and amp glow are deterministic, a dark frame captures them without any scene signal, and subtracting it removes them from the real exposure. Astrophotographers shoot and average many dark frames into a master, so that the subtraction adds as little random noise as possible.
Examples in practice
A master dark built from twenty five-minute frames at the same temperature cleans an entire night's deep-sky captures.