Single word for "Someone who's in on a secret"

Is there a noun that means "someone who's in on a secret"? There are words like accomplice and co-conspirator for people who take part in clandestine activities, but I failed to find one for people trusted with a secret, but not actively participating in anything.

Solution 1:

Consider confidant

A person with whom one shares a secret or private matter, trusting them not to repeat it to others: a close confidante of the princess

[ODO]

Solution 2:

Consider,

insider

One who has special knowledge or access to confidential information. American Heritage® Dictionary

initiate

A person who is being formally accepted or who has been formally accepted as a member of a group or organization

(adj.) Instructed in some secret knowledge M-W

repository

A person to whom something is confided or entrusted; He's the repository of many secrets. M-W

Solution 3:

Originally conspirator simply meant someone who breathed with you; but words change and as you say, it desn't mean that now.

Try confidant

Merriam-Webster link.

Solution 4:

The word privy almost meets your requirements, but it is an adjective rather than a noun. I have heard people use privies as noun but I can't verify the correctness of that or not.