RAE
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(DLE) Diccionario de la lengua española
- The official dictionary.
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(DE) Diccionario del estudiante
- The official dictionary “of the student,” but here everything easier to understand definitions, and each entry goes with an example, so I recommend it the most especially to intermediate leraners.
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(DPD) Diccionario panhispánico de dudas
- Contains articles on many things that cause difficulty to native speakers.
- (NGLE) Nueva gramática de la lengua española
- (ODLE) Ortografía de la lengua española
Others
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espanolavanzado
- Grammar- and expression-focused articles.
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wordreference
- A very friendly bilingual dictionary. Contains conjugation tables.
Warning
Many people use Spanishdict. Although there is nothing wrong with it, they don’t verify examples. There is a small and barely visible note “ⓘ Examples have not been reviewed.” above them. If you want to use Spanishdict, make sure to only focus on the examples they provide for the entries, and take examples from the examples tab/section with a grain of salt, as they often contain errors (for example asimísmo have examples with a sí mismo, or si no is often misspelled, since the word sino exists.