Definition of the word carbon copy

The term "carbon copy" originated in the early 20th century when carbon paper was commonly used to make multiple copies of documents. Carbon paper is a thin, transparent sheet coated with a chemical that leaves an impression of ink when pressed against it. When a person typed on a carbon-enabled typewriter, a copy would be made simultaneously as the original document, thanks to the carbon paper between the original and the copy sheet. Thus, anything printed on the original, such as letters, forms, or messages, would also appear on the carbon paper, making a copy. This method was popular until the 1970s when digital technology replaced it with photocopiers and printers.