An index is an object associating constants with sequential arrays of facts or rules containing those constants. For example the object shown below is a "full" index for a dataset consisting of the facts p(a,b) and p(b,c).

EpilogJS uses different types of indexes, e.g. indexes in which sentences are indexed only on the relation, indexes in which sentences are indexed on all constituent constants, and so forth. See flatindex, fullindex, ruleindex, baseindex.