Thesaurus Linguae Latinae (TLL)
The largest and most detailed Latin dictionary in the world, originating in the late 19th century; entries cover the classical period up to ca. 600 AD. Be sure to read the very good TLL User Guide on the “Overview” page. A "Full Text" search will look for your word throughout the text of lexical articles, not just in headwords. Search results can be sorted by relevance, or alphabetically by lemma. The "Outline View" of the articles is the most useful, and includes hyperlinks to the Index Librorum, the list of works from which the article was compiled.
Database of Latin Dictionaries ("AZ")
Enables searching across more than twenty dictionaries that specialize in different periods and types of Latin. Different parts of the dictionary articles -- their headwords, definitions, and textual citations -- may be searched separately.
Dictionary of Medieval Latin from British Sources
Covering the period from Gildas (fl. 540) to William Camden (1600), this dictionary draws from thousands of Medieval Latin texts, mainly those written in Great Britain or written by British authors abroad.
Mediae Latinitatis Lexicon Minus / Niermeyer, Jan Friedrich. Leiden: Brill, 1954-1976. A Medieval Latin-French/English dictionary in two volumes.
Mittellateinisches Wörterbuch bis zum ausgehenden 13 Jahrhundert. Munich: Beck, 1967-. A through I (partial).