Secrecy was the action of keeping something hidden or the state of something being kept hidden. The hidden something was known as a secret.
In 2254, the medical state of the survivors from the crashed SS Columbia was described by Doctor Phil Boyce as being excellent. Doctor Theodore Haskins said there was a reason for their condition, but he was not sure if Earth was ready to learn this information. He gave permission for Vina to show Captain Christopher Pike the secret to their excellent health. (TOS: "The Cage")