Indexing skills have also been applied to periodicals and databases, and web indexing is now becoming important.

In mathematics, an **index** (plural **indices**) is a superscript or subscript to a symbol. Superscript indices are often, but not always, used to indicate powers. Subscript indices are usually used to label a set or sequence of variables. See also index set.

The index of a subgroup is the number of its left cosets (which is equal to the number of its right cosets).

The index of a Fredholm operator is the dimension of its kernel minus the dimension of its cokernel.

The index of a real quadratic form *Q* is defined (but not always consistently) as *p* − *q* where *Q* can be written as a difference of *p* squared linear terms and *q*squared linear terms.

In computer science, an **index** is usually used for expressing an *n*th element. Indices are usually expressed as integers.