Cofactor

The signed version $C_{ij}$ of a Minor $M_{ij}$ of a Matrix

$$C_{ij} \equiv (-1)^{i+j} M_{ij}$$

used in the computation of the matrix's Determinant

$$\mathop{\rm det}\nolimits ({\hbox{\sf A}}) = \sum_i a_i C_{ij}.$$

See also Determinant Expansion by Minors, Minor