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Synonyms for Hidden (same or very similar meaning)

WordNet sense 1 (not accessible to view):
out of sight

WordNet sense 2 (impossible or nearly impossible to see; imperceptible by the eye):
invisible, unseeable

WordNet sense 3 (designed to elude detection):
secret

WordNet sense 4 (difficult to find):
obscure

WordNet sense 5 (hidden on any grounds for any motive):
concealed


From the ODE community, based on WordNetadd/edit