Blind Carbon Copy
Bcc is defined in RFC 822 and supported by most e-mail systems. A normal, non-blind "CC" header would be visible to all recipients, thus allowing them to reply to each other as well as to the sender. According to RFC 822, the addresses listed in a BCC header are not included in the copies of the message sent to the recipients. RFC 822 says BCC addresses may appear in the copy sent to "BCC" recipients themselves (though this would be unusual).