A deficiency is a lack of something necessary — and in nutrition the word almost always means a nutrient deficiency, the state where intake or absorption of a vitamin, mineral, essential fatty acid, or amino acid falls short of the body’s needs. Mild deficiency dulls function: low iron saps energy and concentration, low vitamin D weakens bone. Severe or prolonged deficiency produces the classic deficiency diseases — scurvy from lack of vitamin C, rickets from lack of vitamin D, goitre and cretinism from lack of iodine. Causes run from poor diet and poverty to malabsorption, increased needs in pregnancy, and drug-nutrient interactions. Public health fights deficiency with fortification, supplementation, and dietary diversification. The concept is foundational: much of nutrition science is the study of what happens when the body does not get enough.