A length of blank paper at the beginning of a fresh roll that an operator feeds through the various rollers, cylinders, and tensioners of a web press. Threading the leader establishes the paper path before the press is brought up to speed and printing begins. It is entirely waste material and is discarded once the press is running smoothly.
Heard on the job
Tape the new roll to the old web and use it as a leader to pull the paper through the drying oven.