Aka “Optical Transfer Function”
OTF describes how well an imaging system creates an image. Part of OTF includes MTF, dealing with contrast and resolution (see MTF definition above). MTF answers the question of: how black is black and how white is white in small details on an image? Another part of OTF deals with how an imaging system messes with phase in the “PTF” or phase transfer function. If phase gets messed up, an image can show a shift in the position of patterns it takes an image of.
See longer article for more information: HERE.