A subset of a vector space , with the inner product , is called orthogonal if when . That is, the vectors are mutually perpendicular.

Note that there is no restriction on the lengths of the vectors. If the vectors in an orthogonal set all have length one, then they are orthonormal.

The notion of orthogonal makes sense for an abstract vector space over any field as long as there is a symmetric quadratic form. The usual orthogonal sets and groups in Euclidean space can be generalized, with applications to special relativity, differential geometry, and abstract algebra.