Merriam-Webster Collegiate Dictionary - cofactor

 noun  Date: 1885  1. the signed minor of an element of a square matrix or of a determinant with the sign positive if the sum of the column number and row number of the element is even and with the sign negative if it is odd  2. a substance that acts with another substance to bring about certain effects; especially coenzyme  3. something (as diet or a virus) that acts with or aids another factor in causing disease