Vitamin D: any of a group of vitamins found in liver and fish oils, essential for the absorption of calcium and the prevention of rickets in children and osteomalacia in adults. They include calciferol ( vitamin D2 ) and cholecalciferol ( vitamin D3 ). Vitamin D is important for good overall health and strong and healthy bones. It’s also an important factor in making sure your muscles, heart, lungs and brain work well and that your body can fight infection. Your body can make its own vitamin D from sunlight. You can also get vitamin D from supplements and a very small amount comes from a few foods you eat.