在数学中，集合 A 的元素有时可以凭借某个集合 J 来索引（index）或标定（label），这时便称集合 J 为索引集。索引由从 J 到 A 的一个满射函数构成，而被索引的搜集称为索引族、标记族或加标族，通常写为(Aj)j∈J。
英语百科
Index set 指标集
In mathematics, an index set is a set 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 a set J, then J is an index set. The indexing consists of a surjective function from J onto A and the indexed collection is typically called an (indexed) family, often written as (Aj)j∈J.