A deficiency's a lack of something... Example : there's a deficiency of oxygen in the air. In mathematics, a deficient number's a number n for which σ(n) < 2n. In medicine there're a variety of nutrient deficiencies: Avitaminosis's a deficiency of vitamins. Boron deficiency (medicine) Chromium deficiency Iron deficiency (medicine) Iodine deficiency Magnesium deficiency (medicine) Micronutrient deficiency In construction, a deficiency's an item, or condition that's considered sub-stan… (More on Deficiency)