Coindexing

In generative syntax, coindexing refers two elements in a structure with the same referential index.

Comments
Coindexing may come about in different ways. Firstly, when an element undergoes movement, all elements in the resulting chain (the moved element itself, and any traces it has left) bear the same index by convention. Alternatively, two NPs may be assigned the same referential index, indicating either that they share the same reference (in case the NPs in question may be interpreted as referential expressions) or that one (typically a pronominal, an anaphor, or an anaphoric epithet) is interpreted as a bound variable (in case the NP it is coindexed with is interpreted as an operator). The former case of coindexing plays a crucial role in the definitions of chain and antecedent government. The latter case is relevant to binding theory.

Link
Utrecht Lexicon of Linguistics