What is inverted file in PostgreSQL?

In PostgreSQL, an Inverted file is basically an index data structure used for mapping content to its location to a database file, within a document, or in sets of documents.

It is usually composed of all the distinct words found in a text and a list containing the occurrences of a word in the text.

The inverted file is widely used in a data structure for document retrieval systems in supporting a full-text search.