A form of vitamin D that helps the body use calcium and phosphorus to make strong bones and teeth. It is fat-soluble (can dissolve in fats and oils) and is found in plants and yeast. It can be made in the body from another form of vitamin D when the body is exposed to the sun. Vitamin D2 is also made in the laboratory. It is used to prevent and to treat vitamin D deficiency. It is a type of dietary supplement. Also called ergocalciferol.