As for Portuguese from Portugal, there are some top quality websites for monolingual dictionaries of the language. One of the best is Infopédia: Dicionários Porto Editora, a very comprehensive online dictionary that is easy to access and use, and also includes sections dedicated to orthography, medical terms, acronyms and abbreviations, and even Portuguese for foreigners.
Here’s the URL: https://www.infopedia.pt/dicionarios/lingua-portuguesa
On the other hand, if you need a Brazilian Portuguese reference, one of the top institutions online is Dicio: Dicionário Online de Português. Here you will find sections with the most searched words in the dictionary, synonyms to help you enrich your vocabulary, and even a section to filter words that contain a certain letter, as well as answering any doubts and curiosities of the Portuguese language.