There is not enough of whatever the subject is. For example, one who is iron deficient does not have enough iron in their body and should probably take an iron supplement.
The term mineral deficiency means a condition where the concentration of any one of the minerals essential to human health is abnormally low in the body.
noun Biochemistry .the oxygen-carrying pigment of red blood cells that gives them their red color and serves to convey oxygen to the tissues: occurs in reduced form (deoxyhemoglobin) in venous blood...