When the writer of a letter sends a copy to a third party without the knowledge of the person receiving the original letter, it is referred to as a:

Blind copy

Explanation:

Blind copy is the term used to describe a situation in which the same message, email, or letter is sent to several different people, without these people knowing about each other, or knowing about those copies. In short, this term refers to when the writer of a letter sends a copy to a third party without the knowledge of the person receiving the original letter. It can also be called Blind carbon copy or Blind Courtesy Copy.