What is Carte Blanche?

Legal Definition
The signature of an individual or more, on a while. paper, with a sufficient space left above it to write a note or other writing.

2. In the course of business, it not unfrequently occurs that for the sake of convenience, signatures in blank are given with authority to fill them up.. These are binding upon the parties. But the blank must be filled up by the very person authorized. 6 Mart. L. R. 707. Vide Ch. on Bills, 702 Penna. R. 200. Vide Blank.
-- Bouviers Law Dictionary
Legal Definition
A white sheet of paper; an instrument signed, but otherwise left blank. A sheet given to an agent, with the principal's signature appended, to be flli-ed up with any contract or engagement as the agent may see fit. Hence, metaphorically, unlimited authority.
-- Black's Law Dictionary
Legal Definition
A signed blank instrument to be filled in and used by another without restriction. See 2 Pa. St. 200, 44 Am. Dec. 190.
-- Ballentine's Law Dictionary