The Free On-line Dictionary of Computing (30 December 2018):

Blind Carbon Copy

    (BCC) An electronic mail header which lists
   addresses to which a message should be sent, but which will
   not be seen by the recipients.

   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).

   (1998-03-14)