In email, bcc means you are sending a hidden copy. People in the To and cc lines cannot see the bcc names, and the bcc people usually cannot see one another either. You use it when someone should know about the mail without showing their address to the whole group, or when you write to many people and do not want to leak everyone’s email. It is office language; mail programs print it as bcc or Bcc. Though actually, nobody is using carbon paper any more — the name is a leftover from old offices, where a “carbon copy” was a spare made by sliding carbon paper under a letter, and “blind” meant the others were kept in the dark about who got that spare. Putting someone in bcc does not make the mail secret in every other way; it only hides that name from the list the others can see.