I forgot this word. I tell a person a secret and ask him not to tell it to anyone else. That 2nd person tells another person and tells him not to disclose it to anyone else. But this goes on.
Important thing here is that, the info should be kept secret. But he assumes he is only disclosing this to another person (may be a close friend or spouse) and that that info will not go out. But the other person assumes the same and passes on to his close friend or spouse or someone s/he most trusts, etc..
Note that, the info passes from one person to another person. Not group or mass public.