A group of related documents aginst which information retrieval is employed is called*repositoryCorpusindex collectionText Corpus

The term that refers to a group of related documents against which information retrieval is employed is commonly known as a corpus.

In the context of information retrieval, a "corpus" is a collection of texts that are used for linguistic research, text analysis, or information retrieval tasks. It's essential to have a well-defined corpus to evaluate search algorithms, natural language processing tasks, and other related fields.

Options Analysis:

1. Repository - This is a general term for a place where data is stored, but it's not specifically aimed at a group of related documents for information retrieval.
2. Corpus - This is the correct answer, as it specifically denotes a body of related texts used in linguistic and computational tasks.
3. Index - This refers to the data structure used to improve the speed of data retrieval operations on a database.
4. Collection - While this could imply a group of documents, it lacks the specificity that "corpus" provides in the context of information retrieval.

Final Answer

The correct term is Corpus.