Coreference Resolution
Co-reference resolutionResolution (CR) is the task of deciding whether two entity mentions refer to the same instance or not.
For example in:
Joe Biden appeared at the event at 8pm. The president was wearing a Louis Vuitton Tuxedo.
The objective is to identify that Joe Biden and The president are the same entity.
Coreference Detection is related to Relationship Extraction (RE) - in fact you could even say that it'sCR is a special case of RE in the sense that we are interested in the special relationship between entity mentions when they both refer to the same entity.
In-Document Coreference Resolution
This is the "normal" CR case in which you're trying to resolve mentions of entities within the same document e.g. a single news article.
Approaches
Cross-Document Coreference Resolution
Cross-Document Coreference Resolution (CDCR) is when you try to link named entity references across multiple input documents. A use case might be identifying that a number of news articles do actually refer to the same person (e.g. "Joe Biden", "The President").
CDCR is challenging because there are so many possible entities and thus O(n2) comparisons to make between candidates.