In algebraic geometry, a morphism f: X → Y of schemes is a finite morphism if Y has an open cover by affine schemes
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such that for each i,
is an open affine subscheme Spec Ai, and the restriction of f to Ui, which induces a ring homomorphism
makes Ai a finitely generated module over Bi. One also says that X is finite over Y.
In fact, f is finite if and only if for every open affine open subscheme V = Spec B in Y, the inverse image of V in X is affine, of the form Spec A, with A a finitely generated B-module.
For example, for any field k, the morphism from the affine line A1 over k to itself given by x ↦ x2 is finite. (Indeed, the polynomial ring k[x] is finitely generated as a module over k[y] by y ↦ x2, with generators 1 and x.) By contrast, the inclusion of A1 − 0 into A1 is not finite. (Indeed, the Laurent polynomial ring k[y, y−1] is not finitely generated as a module over k[y].)
Properties of finite morphisms
Morphisms of finite type
For a homomorphism A → B of commutative rings, B is called an A-algebra of finite type if B is a finitely generated as an A-algebra. It is much stronger for B to be a finite A-algebra, which means that B is finitely generated as an A-module. For example, for any commutative ring A and natural number n, the polynomial ring A[x1, ..., xn] is an A-algebra of finite type, but it is not a finite A-algebra unless A = 0 or n = 0.
The analogous notion in terms of schemes is: a morphism f: X → Y of schemes is of finite type if Y has a covering by affine open subschemes Vi = Spec Ai such that f−1(Vi) has a finite covering by affine open subschemes Uij = Spec Bij with Bij an Ai-algebra of finite type. One also says that X is of finite type over Y.
For example, for any natural number n and field k, affine n-space and projective n-space over k are of finite type over k (that is, over Spec k), while they are not finite over k unless n = 0. More generally, any quasi-projective scheme over k is of finite type over k.
The Noether normalization lemma says, in geometric terms, that every affine scheme X of finite type over a field k has a finite surjective morphism to affine space An over k, where n is the dimension of X. Likewise, every projective scheme X over a field has a finite surjective morphism to projective space Pn, where n is the dimension of X.