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:

It is referred to as a Blind Carbon Copy (BCC).

Explanation:

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 Carbon Copy. When the writer sends the letter to another person and the receiver of the original letter know about it, the letter is referred to as the Carbon Copy, but when the receiving person doesn't know about it, it is referred to as a Blind Carbon Copy.

This concept is used in emails as well. When you send email to a person, there are options like CC (Carbon Copy) and BCC ( Blind Carbon Copy).  

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