A sequence of (key, pointer) pairs where each pointer points to a record in a database which contains the key value in some particular field. The index is sorted on the key values to allow rapid searching for a particular key value, using e.g. binary search. The index is "inverted" in the sense that the key value is used to find the record rather than the other way round. For databases in which the records may be searched based on more than one field, multiple indices may be created that are sorted on those keys.
An index may contain gaps to allow for new entries to be added in the correct sort order without always requiring the following entries to be shifted out of the way.