A "flat dark" is a dark frame that has the same exposure time and temperature as the flat-field image. Often flats are taken at much shorter exposure times than your science frames, and so you might use special dark frames for subtraction. A regular "dark frame" just matches the science frame - it may be a stack of darks with the same exposure time, or it may be a longer exposure stack that is scaled to match the exposure time of the science images.
A "flat dark" is a dark frame that has the same exposure time and temperature as the flat-field image. Often flats are taken at much shorter exposure times than your science frames, and so you might use special dark frames for subtraction. A regular "dark frame" just matches the science frame - it may be a stack of darks with the same exposure time, or it may be a longer exposure stack that is scaled to match the exposure time of the science images.
Arne