The dictionary is a powerful tool for learning Latin. After the grammar book, this is the most important resource for expanding your knowledge of the Latin language; once you reach intermediate-plus level in the Latinum Institute programme, it is time to start relying on Latino-Latinum resources as much as possible, to 'keep your head in Latin' and simulate to the degree possible in a world where there are no Romans, an immersive environment.
In the list of Latin books below you will find generalist dictionaries, and a second list of specialist subject-specific dictionaries for areas such as philosophy, zoology and chemistry.
Once a student reaches Intermediate-Plus level, they should be ready to move from a Latin-Vernacular dictionary to a Latin-Latin one. Unfortunately, there is currently no suitable student level Latin-Latin dictionary in print: we do have a number of dictionaries and thesauri that give Latin definitions that are useful for students, however, and I highly recommend spending time reading these, once your Latin is good enough. Dictionary reading is an excellent way to expand your knowledge of Latin.
Anon. Manulae Vocabulorum This dictionary gives very brief definitions of words in Latin - written for school pupils, this comes closest to what a student might require from a basic Latin-Latin schoolroom dictionary that I have seen so far.
Boccardo Vocabularium Pyladae Brixiani
Comenius - Lexicon Januale: digitised by Evan der Millner. This is not strictly speaking a dictionary.
Dolet Formulae Latinarum Locutionum Illustriorum by Stephano Doleto Gallo Aurelio has a selection of definitions and phrases.
Étienne Thesaurus (Some students have found this to be a practical alternative.)
Fontanus Hortulus Puerorum
Forcellini Totius Latinitatis Lexicon
Gesner Novus Thesaurus
Gradus ad Parnassum - not strictly a dictionary, but it gives synonyms and antonyms, so often enough detail to work out what a word means, assuming you already have a baseline vocabulary.
Nizzoli Lexicon Ciceronianum
Serra Franciscus: Latin Thesaurus with short comprehensible Latin definitions. This pdf edition also has a gloss in German.
Trebellio Latinae Linguae Universae Promptuarium Pars Prima Pars Altera
Wagner Lexicon Latinum
SPECIALIST VOCABULARIES (LATINO-LATINUM)
Anon. Lexicon Juridicum
Gessner Icones Animalium Quadrupedum
Gessner Historia animalium: De Quadrupedibus Oviparis, Volume 2
Gockel Lexicon Philosophicum
Johnson Lexicon Chymicum
Kentmann Nomenclaturae Rerum Fossilium
Napolitani Lexicon Universi Corporis Juris Volumen Secundum
Pierruges Glossarium Eroticum Linguae Latinae
Signoriello Lexicon Peripateticum