Here you can test and improve your Latin vocabulary. The exercise is highly customizable: you can focus on a specific group of words (nouns of each declension, adjectives, verbs, adverbs), or on all of them together; and you can select three different vocabulary sizes (small, around 250 words; medium, 600; large, 1200).
I did my best to select the most appropriate meaning for each word, but of course this is certainly not a full dictionary. If you need an online dictionary, you might want to consider the Elementary Latin dictionary by C.T. Lewis or the Latin Dictionary by C.T. Lewis and C. Short: they are rather old, but still usable and freely available in the Perseus Digital Library.