(business) The authority of an employee acting unilaterally to commit an employer to a contract.
1996, Andrew Turnauer, Navigating the Mortgage Maze, page 124:
Some small banks still use this model, but larger institutions tend to entrust a single individual with signature authority, at least to a certain loan dollar amount.
As a first level manager my signature authority is $500.
(law) Control over a financial account with ones signature.
2019, “Understand how to report foreign bank and financial accounts”, in Internal Revenue Service:
Officers or employees who file an FBAR to report signature authority over an employer’s foreign financial account don’t need to personally keep records on their employer’s accounts.