I'll term it as a software technique in which we knowingly inject the fault, this can be either a software or hardware fault injection. But we'll talk here for the Software Fault Injection.
If this then What is the Difference between Fault injection and Negative testing ?
Negative testing is when you are testing to ensure that the AUT does NOT perform certain actions.
Fault injection is when a defect or anomolie is intentionally inserted into an AUT and the test cases task is to then ensure that the coverage is such that the fault is discovered. This is a test of the cases and plans for coverage completeness.
One could insert a fault that means that a negative test case needs to be created and executed in order for this fault to be discovered, but a positive test case fault might also be inserted.