Subject indexing is the act of describing or
classifying a
document by
index terms or other symbols in order to indicate what the document is
about, to summarize its
content or to increase its
findability. In other words, it is about identifying and describing the
subject of documents. Indexes are constructed, separately, on three distinct levels: terms in a document such as a book; objects in a collection such as a library; and documents (such as books and articles) within a field of knowledge.