Matrix
A matrix is a rectangular array of scalars from a field , arranged in rows and columns. An matrix has rows and columns.
Every linear map between finite-dimensional vector spaces can be represented by a matrix, once bases are chosen. Matrix multiplication corresponds to composition of linear maps. The set of matrices over forms a (generally non-commutative) ring.
A square matrix is invertible if and only if its determinant is nonzero, equivalently if the corresponding linear map is bijective.