A determinant is a scalar value calculated from a square matrix. Determinants are useful for various purposes, such as finding the inverse of a matrix, solving systems of linear equations, and determining whether a matrix is invertible or not. The determinant of a 2x2 matrix can be calculated as follows:

| a  b |
| c  d | = ad - bc

For example, given the matrix:

[ 2  3 ]
[ 4  6 ]

The determinant is calculated as (26) - (34) = 0. Determinants have many properties and rules that allow for efficient calculations and transformations.