- X × Y → Y
is a closed map, i.e. maps closed sets onto closed sets. This can be seen as an analogue of compactness in algebraic geometry: a topological space X is compact if and only if projection along X is a closed map in any product X × Y, equipped with Tychonoff product topology.
The most common example of a complete variety is a projective variety, but there do exist complete non-projective varieties in dimensions 2 and higher. The first examples of non-projective complete varieties were given by Masayoshi Nagata and Heisuke Hironaka. An affine space of positive dimension is not complete.
The morphism taking a complete variety to a point is a proper morphism, in the sense of scheme theory. An intuitive justification of 'complete', in the sense of 'no missing points', can be given on the basis of the valuative criterion of properness, which goes back to Claude Chevalley.
- Section II.4 of Hartshorne, Robin (1977), Algebraic Geometry, Graduate Texts in Mathematics 52, New York: Springer-Verlag, ISBN 978-0-387-90244-9, MR 0463157
- Chapter 7 of Milne, James S. (2009), Algebraic geometry, v. 5.20, retrieved 2010-08-04
- Section I.9 of Mumford, David (1999), The red book of varieties and schemes, Lecture notes in mathematics 1358 (Second, expanded ed.), Springer-Verlag, doi:10.1007/b62130, ISBN 978-3-540-63293-1