a data structure or collection of supporting data structures, used in many information retrieval system to speed up processing. Each term (word or stem or root or phrase or descriptor or concept ...) has an entry (often found by hashing, through a trie, or with a B-tree) which points to all the occurrences of that term in the collection. Sometimes the entry points to a short record that contains the number of occurrences and a pointer to where the list of occurrences begins, in another data structure. Often the occurrences indicate documents, though they may identify every match inside any document.