[mrkeen]

My working definition is that a unit is something which can be unit-tested. By that I mean: the code being tested has the same properties that you expect of a good unit-test. I guess fast and deterministic are good enough properties for a working definition.

writeFile(fname,"Hello, World") is only one thing, but its behavior will depend on the state of the filesystem, so it's not a unit.

parseComplicatedObject(bytes) could be a unit (even if it calls out to many other sub-parsers - as long as they are also units).

One thing I see in a lot of companies is efforts to reduce test flakiness. Devs will attempt this work in src/test. But if the code is flaky, and you change the test from flaky to reliable, then you have just decreased the realism of your test. Like I mentioned with my comment above, you reduce test flakiness by doing the hard work in src/main. The src/test changes should be easy after that.