I'd actually quibble a lot on this definition--if you want to unit test code that needs to do any of the first three things, well, you have to do them to test t

I'd actually quibble a lot on this definition--if you want to unit test code that needs to do any of the first three things, well, you have to do them to test the code. I would say that a test for a network protocol that works by spinning up an echo server on a random port and having the code connect to that echo server is still a unit test for that network protocol. In my definition, it would still be a unit test if it is fast and it talks to a database, a network server, or the file system, so long as such communication is done entirely in a "mock" fashion (it only does such communication as the test sets up, and it's done in such a fashion that tests can be run in parallel with no issues).