carbon paper

from The American Heritage® Dictionary of the English Language, 5th Edition.

• 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.

from The Century Dictionary.

• noun Paper faced with carbon or lampblack: used between two sheets of paper for the purpose of reproducing upon the lower sheet anything which may be written or drawn upon the upper sheet, or printed upon it by a type-writer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sheet of paper with one of the faces impregnated with carbon, used to make carbon copies.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a thin paper coated on one side with a dark waxy substance (often containing carbon); used to transfer characters from the original to an under sheet of paper