What is "billet reading"

n. A magic trick or form of clairvoyance in which the performer reads a message sealed inside an envelope.

Billet reading, or the envelope trick, is a mentalist effect in which a performer pretends to use clairvoyance to read messages on folded papers or inside sealed envelopes. It is a widely performed "standard" of the mentalist craft since the middle of the 19th century. Billet is the French term for note or letter, referring to the rectangular shape of the paper.