Definition
Coindexation occurs when noun phrases (NPs) refer to the same entity in the world and corefer. As a result, two NPs that are coindexed receive the same index. An index most commonly is a letter or number, but may be anything to indicate NPs coreference. (Carnie, 2006, p. 150)
Examples

References
Carnie, A. (2006). Syntax: A generative introduction. John Wiley & Sons.