EN
Any research on the vocabulary of medieval languages can rest on dictionaries since they offer an organized material with examples: their transfer on digital device increased their value by raising new questions in chronological or onomasiological terms. However, we should also admit its limits and weaknesses: within the framework of a semantic study, their consulting should not dispense with a systematic and thorough investigation into the texts, which is made easier and more fruitful thanks to the digitalised corpora. The analysis we have carried out about the notion of treasure in the Middle Ages from the observation of contextual uses of the words 'thesaurus' and 'tresor' has led us to make sure that the conditions to write the dictionaries do not always allow the editors to avoid anachronisms and that, besides, the lexicographical entries do not provide a satisfactory framework to give an in-depth account of a complex semantic system.