Italy's most authoritative monolingual dictionary, free online with etymologies.
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The reference Italians themselves reach for: full definitions, etymology, usage labels and a synonyms section, alongside Treccani's encyclopedia. Switching to a monolingual dictionary is one of the biggest jumps you can make at B1+, and this is the one to switch to. Entries are dense and written in fairly formal Italian, so keep a bilingual dictionary around for a while; some extras and the ad-free app are paid.