1. A method of indexing multimedia documents, the method being characterized in that it comprises the following steps: a) for each document, identifying and extracting terms t i constituted by vectors characterizing properties of the multimedia document for indexing, such as shape, texture, color, or structure of an image, the energy, the oscillation rate or frequency information of an audio signal, or a group of characters of a text; b) storing the terms t i characterizing the properties of the multimedia document in a term base comprising P terms; c) determining a maximum number N of desired concepts combining the most pertinent terms t i , where N is an integer less than P, with each concept c i being designed to combine all terms that are neighboring from the point of view of their characteristics; d) calculating the matrix T of distances between the terms t i of the term base; e) decomposing the set P of terms t i of the term base into N portions P j (1≦j≦N) such that P=P 1 ∪ P 2. .. ∪ P j. .. ∪ P N , each portion P 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 the leaves contain the concepts c i of the dictionary and the nodes of the tree contain the information necessary for scanning the tree during a stage of identifying a document by comparing it with previously-indexed documents; and g) constructing a fingerprint base made up of the set of concepts c i representing the terms t i of the documents to be indexed, each document being associated with a fingerprint that is specific thereto.