Defect means the lack or imperfection of something which leads to failure or inadequacy. It also has a meaning of abandon a position in order to join another, opposing group. For example, if a car can not run properly or has faulty brakes, than it has a defect. Deficiency also has a meaning of some kind of imperfection or lack, but mainly in relation to health. For example, colour vision deficiency means colour blindness, or any deviation of the normal perception of colours.
