This is a really useful resource in French placed online by the French National Centre for Scientific Research. I find that it’s especially good when searching for rarer words, or words that are no longer often used. Enjoy!

“Créé en 2005 par le CNRS (Centre national de la recherche scientifique), le Centre national de ressources textuelles et lexicales (CNRTL) fédère au sein d’un portail unique, un ensemble de ressources linguistiques informatisées et d’outils de traitement de la langue.”

Consult the portal here: http://www.cnrtl.fr/definition/portail

Once you have searched for a word, click on the tabs above the search box to see the relevant information on: morphology, lexicography, etymology, synonymy, antonymy, and concordance.

In the lexicography section of the lexical portal, you can also click on the left-hand sidebar to see different definitions from around seven dictionaries (TLFI, Académie, BDLP, BHVF, DMF).

The website also contains access to a wealth of French dictionaries, as well as a French/Portuguese dictionary of idiomatic expressions. Click on “dictionaries anciens” or “dictionaries modernes” from the “Dictionnaires” tab to access the lists: http://www.cnrtl.fr/dictionnaires/anciens/