1. A method of indexing multimedia documents, the method being characterized in that it comprises the following steps: a) for each multimedia document, identifying and extracting terms t i constituted by vectors characterizing properties of each of the multimedia documents for indexing, which characterizing properties include at least one of shape, texture, color, and structure of an image, an energy level, an oscillation rate or frequency information of an audio signal, and a group of characters of a text; b) storing the terms t i characterizing the properties of each of the multimedia documents in a term base comprising a set of P terms, where P is an integer; c) determining a maximum number (N) of desired concepts combining pertinent terms t i , where N is an integer less than P, with each concept c i being designed to combine all terms t i based on a relationship of their characterizing properties; d) calculating a matrix T of distances between the terms t i of the term base of P terms; e) decomposing the set of P terms into N portions P j (1≦j≦N) such that the set of P terms=P 1 ∪ P 2. .. ∪ P j. .. ∪ P N , each portion P j comprising a set of terms t ij and being represented by a concept c j , the terms t i being distributed in such a manner that terms that are farther away are to be found in distinct portions P l , P m while terms that are closer together are to be found in the same portion P l ; f) structuring a concept dictionary so as to constitute a binary tree in which each leaf of the binary tree contains a concept c i of the dictionary and each node of the binary tree contains information necessary for scanning the tree during a stage of identifying a multimedia document by comparing it with previously-indexed documents; and g) constructing a fingerprint base made up of a set C of concepts c i representing the terms t i of the multimedia documents to be indexed, each multimedia document being associated with a fingerprint that is specific thereto.