---
tag: RHF0MXBMBT
kind: concept
name: Index set
aliases: [family-set-theory, index-sets, indexing-set]
sources:
  - wikipedia: en:393055@r1350537554
    locator: Index set
    note: adopted
    mention: wikipedia://en/393055
    sections: [Summary]
same_as:
  wikidata: Q21285678
  wikipedia: Index set
inventory: [7MD5K07DB7]
---

## Summary

In mathematics, an **index set** is a [[set|0BSJ7RQ9BW]] whose members label (or index) members of another set. For instance, if the elements of a set $A$ may be *indexed* or *labeled* by means of the elements of a set $J$, then $J$ is an index set. The indexing consists of a [[surjective function|01RVCMSN5N]] from $J$ onto $A$, and the indexed collection is typically called an *indexed family*, often written as $\{A_{j}\}_{j\in J}$.