You are proficient in a language not when you can translate it into your own, but when you no longer need to.
As a historian of Christianity from roughly Augustine (4th century) to early modern Europe (17th century), I find myself reading a lot of Latin. Call it immersion language learning via the or-else-I-don’t-keep-my-job method. One of the most time-consuming and often unpleasant tasks is vocabulary acquisition. In order to read Latin efficiently, you need a base of about 1,500 words. That takes you to about 80% recognition, meaning you still have to guess or look up 1 out of 5 words. (By the way, the vocab lists by the folks at Dickinson college are invaluable for prioritizing vocabulary.) That means that any Latinist is going to spend quite a bit of time with dictionaries.
That’s dictionaries emphatically in the plural, since the chronological extent and geographical scope of Latin make specialized works a necessity. Anthony Grafton, one of the world’s most respected Renaissance scholars, once in an interview described his work space as filled with “the many dictionaries I depend on for reference.” As you can see, he’s not kidding.
Recently I’ve found a dictionary that not only gives me the answers I seek, usually on the first attempt, but also assists me in my goal of vocabulary acquisition through language immersion. Egidio Forcellini’s Totius latinitatis lexicon is a massive, monolingual dictionary compiled by an 18th-century Italian philologist and periodically revised since. Monolingual means that, unlike your usual Latin-English or Latin-French lexicons, this dictionary gives its definitions in Latin. This may make the work more difficult to use for beginners, but it is imperative for intermediate or more advanced students of a language to get away from bilingual dictionaries as soon as possible. All of that extra time spent in the language, without “breaking character,” is essential for developing the ability to think in and use the language. After all, omnia dicta fortiora, si dicta latina (everything sounds more impressive in Latin).
All the entries I’ve looked at so far have been quite helpful. In fact, I’d say that the entries are clearer than those in Lewis & Short. It may be a good bit more comprehensive as well; my unscientific survey showed over 400 pages of A’s in Forcellini to just over 200 in L&S (the total page count is also significantly larger). Also, it’s not actually entirely in Latin. The definitions are, but it often additionally provides glosses in a number of languages both modern and ancient, so it’s really the best of both worlds.
There are a few ways to access Forcellini. The best is through the Database of Latin Dictionaries, which you may have access to if you have library privileges at a serious liberal arts university. The layout is very clean, it’s fully searchable, and of course there are other dictionaries for comparison. Almost as good, and available to anyone with internet access, is the searchable Forcellini at Linguax. The search brings up digitized page images, which are not quite as neat as the DLD layout, but still easy enough to read. Finally, you could download the PDFs from Documenta Catholica Omnia, but I find that solution bulky and slow.
Right now, Forcellini is my shiny new toy. We’ll see how it holds up. For quick glossing, I use the handy program Whitaker’s Words. Classical works comparable to Forcellini are L&S and Oxford Latin Dictionary. For specifically Christian terminology, I use Blaise’s Lexicon latinitatis medii aevi. For more general medieval Latin, I usually turn to Niermeyer’s Mediae latinitatis lexikon minus, but I’m impressed with Latham’s Revised Medieval Latin Word-List from British and Irish Sources. For Renaissance and modern Latin, there’s the Vatican’s Lexicon recentis latinitatis. Maybe I should ask Grafton where to get a dictionary wheel.