Negative Test Definitions

"Negative testing ensures that the plot of the application is according to the requirements and can handle the unwanted input and user behavior[sic]. In this testing we put invalid data and see the output against the given input.We determine that application is not doing anything that it isn't supposed to do. Negative testing is also known as failure testing or error path testing. When performing negative testing exceptions are expected. This shows that your application is able to handle improper user behavior[sic]."
Wikipedia

"Validating the application against invalid data"
http://www.softwaretestinghelp.com/what-is-negative-testing/

"Negative Testing is a variant of testing that can be performed on the system by providing invalid data as input."
https://www.guru99.com/positive-vs-negative-testing.html

"Negative testing is probably the most hated category of tests by development: the equivalent of whining. In code, this is handled by checking for every kind of wrong input the user can possibly supply, and a lot of developers consider this to be somebody else's problem: either a user problem, or the problem of the UI client. Nothing could be further from the truth. Catching bad inputs before they hit the business logic, or worse the database, makes perfect sense."
https://www.soapui.org/testing-dojo/best-practices/negative-testing.html

"A check to see if some part fails correctly"