The official monolingual Spanish dictionary from the RAE and the 22 language academies.
Details
The authority on what a Spanish word means, including regional labels showing which country a sense belongs to — the thing to reach for when a bilingual dictionary gives you three English words and no way to choose. Free, ad-free and continuously updated online. Definitions are written for native speakers in dense academic Spanish, so it is not a beginner tool; use SpanishDict until monolingual definitions stop being harder than the word itself.