What is a "test harness"? [duplicate]

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• What is test harness? 4 answers

For a Java project is a "test harness" one of the following?

• the junit framework + coded test cases + all required resources to run the tests
• a separate component that acts as a client and invokes application code externally?
  • when I say separate I mean separate from application code and junit test code
• something else I've missed?

I'm just confused about what people mean when they say:

"Please use a test harness for your code"