- any one of two or more factors, the combined effects of which may cause a certain result or condition, esp. a diseasealso written co-factor
- Biochem. an accessory substance, esp. a coenzyme, that must be present for a particular biological reaction to occur
- One of two or more contributing factors.
- A substance, such as a metallic ion or coenzyme, that must be associated with an enzyme for the enzyme to function.