Given a field$k$, the general linear group$GL(n,k)$ (or $GL_n(k)$) is the group of invertible linear transformations of the vector space $k^n$. It can be canonically identified with the group of $n\times n$matrices with entries in $k$ having nonzero determinant.

This group can be considered as a (quasi-affine) subvariety of the affine space$M_{n\times n}(k)$ of square matrices of size $n$ defined by the condition that the determinant of a matrix is nonzero. It can be also presented as an affine subvariety of the affine space $M_{n \times n}(k) \times k$ defined by the equation $\det(M)t = 1$ (where $M$ varies over the factor $M_{n \times n}(k)$ and $t$ over the factor $k$).

This variety is an algebraic $k$-group, and if $k$ is the field of real or complex numbers it is a Lie group over $k$.

One can in fact consider the set of invertible matrices over an arbitrary unital ring, not necessarily commutative. Thus $GL_n: R\mapsto GL_n(R)$ becomes a presheaf of groups on $Aff=Ring^{op}$ where one can take rings either in commutative or in noncommutative sense. In the commutative case, this functor defines a group scheme; it is in fact the affine group scheme represented by the commutative ring $R = \mathbb{Z}[x_{11}, \ldots, x_{n n}, t]/(det(X)t - 1)$.

The above is sometimes referred to as the unstable general linear group, whilst the result if one lets $n$ go to infinity is called the stable general linear group of $R$, and is then written $GL(R)$ with no suffix.