In category theory, a branch of mathematics, a diagram is the categorical analogue of an indexed family in set theory. Wikipedia

Categorical analogue of an indexed family in set theory Wikipedia Disambiguation

Concept of category theory Wikidata

A functor from an index category to another category. The objects and morphisms of the index category need not have any internal substance, but rather merely outline the connective structure of at least some part of the diagram's codomain. If the index category is J and the codomain is C, then the diagram is said to be "of type J in C". Wiktionary