Learner definitions plus bilingual entries in dozens of languages, all free.
Details
The other great free learner's dictionary, distinguished by pairing every English entry with a translation into your own language — genuinely useful when a monolingual definition leaves you unsure — plus CEFR levels, example sentences, both accents in audio and a good grammar section. It is often quicker than Oxford for a rough sense of a word. Coverage of newer slang lags, the bilingual entries are much thinner than the English ones for smaller languages, and the site is ad-heavy.