How many types of dictionaries are there?

Dictionary Typology

This presentation offers the following typology of dictionaries:

• Bilingual/Multilingual (translating one language into another)
• Monolingual
• Synchronic (contemporary usage)
• Limited (a particular field, e.g. medical; a particular register, e.g. slang)
• General: Comprehensive (all of the language, multi-volume) or Standard (single volume, mostly for paedagogical use)
• Diachronic
• Historical (the historical paths that words have taken in their usage and forms)
• Etymological (the origins of words)

Dictionaries often combine several categories. The Oxford English Dictionary is all of Historical, Etymological, and Comprehensive. There are dictionaries that are purely etymological, though historical dictionaries almost always are also etymological. You can also have bilingual dictionaries that are not just translations, but etymological or historical (the big dictionaries of Classical languages fall in that category), or synchronic and limited (e.g. Greek–English medical dictionary).