carbon paper
- very thin paper coated on one side with a waxy, dark-colored preparation, as of carbon: it is placed between two sheets of paper so that the pressure of typing or writing on the upper sheet makes a copy on the lower
- paper used in the carbon process
See carbon paper in American Heritage Dictionary 4
noun A lightweight paper coated on one side with a dark waxy pigment, placed between two sheets of blank paper so that the bottom sheet will receive a copy of what is typed or written on the top sheet.