A very comprehensive monolingual Latin dictionary is Forcellini's Lexicon Totius Latinitatis
in 5-volume, as far as I can see the best monolingual Latin dictionary freely available.
It can be downloaded as a set of PDF-files from the web-site Documenta Catholica OMnia
. One Textkit-user has even created a web-interface
for it (using images of somewhat lower resolution). I can definitely recommend this one.
See also the Texkit-thread Forcellini's 'Lexicon totius Latinitatis'