A more thorough explanation:

Definition: Lettre (le-trə), n. [French "letter"] - A formal document that grants some kind of authority.

Example: In medieval times, a lettre de cachet was a letter signed by the king of France that granted the holder the power to imprison someone without trial.

Explanation: The example illustrates how a lettre can be used to grant authority. In this case, the lettre de cachet was a powerful tool that allowed the holder to imprison someone without due process. This shows how a lettre can be used to give someone significant power over others.