Synonyms of 'hidden'
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You can describe something as abstruse if you find it difficult to understand, especially when you think it could be explained more simply.
Meetings keep reverting to discussions about abstruse resolutions.
Something that is clandestine is hidden or kept secret, often because it is illegal.
They are said to have been holding clandestine meetings for years.
Thesaurus for hidden from the Collins English Thesaurus
Hidden is the past participle of → hide .