Synonyms for Invisible (same or very similar meaning)
WordNet sense 1 (not seen or perceived):
unseen
WordNet sense 2 (hidden and difficult to see):
occult
WordNet sense 3 (not accessible to view):
concealed
WordNet sense 4 (impossible or nearly impossible to see; imperceptible by the eye):
unseeable
WordNet sense 5 (not prominent or readily noticeable):
inconspicuous
From the ODE community, based on WordNet
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