1. One of the contributing causes in any action. 2. One of the components that by multiplication makes up a number or expression. 3. A vitamin or other essential element. 4. An event, characteristic, or other definable entity that brings about a change in health. 5. A categoric independent variable, used to identify, by means of numeric codes, membership in a qualitatively identifiable group; for example, overcrowding is a factor in disease transmission.