Kept or done in secret, often in order to conceal an illicit or improper purpose. See Synonyms at secret.
[Latin clandestnus, probably blend of *clam-de, secretly (from clam; see kel-1 in Indo-European roots) and intestnus, internal; see intestine.
@Pheneks: Neither "clandestiny" nor "clandestinity" appear to be words with definitions (though Clandestiny was a pretty cool video game). Check dictionary.com. But hey, Bloodbath has a song with the fake word "omnious" in the title, so it's not like there's no precendent.