Something not to be told or not known.
An underlying reason.
Secretness.
Separate; hidden; private; secluded; close mouthed.
I wish you could give me your secret. – Opportunities by Susan Warner
My coffin will keep a secret, which yours couldn't do! – The Brightener by C. N. Williamson A. M. Williamson
It was not paid to keep the secret. – The Honour of the Clintons by Archibald Marshall