Calcium is an essential mineral and the most abundant metal in the human body, playing critical roles in bone health, muscle function, nerve signaling, and blood clotting. As a chemical element with atomic number 20, it exists in various forms in food and supplements. In its elemental form, it is a soft, silvery-white alkaline earth metal that reacts with water and oxygen. In dietary contexts, it is obtained from dairy, leafy greens, and fortified foods. Calcium ions are crucial for physiological processes such as enzyme activation and cellular signaling. Deficiency can lead to osteoporosis, while excess intake may cause hypercalcemia. The mineral is vital for maintaining overall health and is commonly supplemented in various bioavailable forms like calcium citrate or carbonate.