What is the term for test data that a program isn't designed to accept?

The correct term for test data that a program isn't designed to accept is invalid data. Invalid data refers specifically to inputs that do not conform to the established rules or constraints of the program, such as data types or acceptable ranges. This type of data is used in testing to ensure that the program can handle situations where incorrect information is entered, and to verify that it behaves as expected under those circumstances.

Erroneous data, while similar, often refers more broadly to data that contains errors, which might include both valid and invalid entries depending on context. Boundary data pertains to values at the edges of acceptable limits and extreme data usually refers to values that are at the very limits of the input range. Therefore, invalid data is the most accurate term for inputs that a program should not accept.