cofactor

Short definition from Wiktionary

Noun

1. 1
   A contributing factor.
2. 2
   A substance, especially a coenzyme or a metal, that must be present for an enzyme to function.
3. 3
   A molecule that binds to and regulates the activity of a protein.
4. 4
   The result of a number being divided by one of its factors.
5. 5
   The signed determinant of the submatrix produced by removing the row and column containing a specified element; primarily used in the recursive definition and calculation of the determinant of a matrix.