【Deficient】 , 【defective】 mean showing lack of something necessary. The words are sometimes used interchangeably though they tend to diverge in their meanings.

【Deficient】 typically implies a falling short in the amount, quantity, or force considered essential to adequacy or sufficiency.

【Defective】 on the other hand typically implies existence of some definite fault, injury, or flaw that impairs the completeness or efficiency of something; thus, a person is 【deficient】 in courage when he has not sufficient courage to meet his difficulties; he is mentally  【deficient】 when he has not sufficient intelligence to enable him to take care of himself; he is mentally 【defective】 (or a mental 【defective】 ) when some fault or defect in his nervous system impairs his ability to think coherently; he suffers from 【defective】 hearing when by disease or injury the organ of hearing has become 【defective】 .