Ask HN: What is the unit in unit testing?

I have been reading a log about testing these days, and luckily have been writing as well. One thing that became clear to me was, testing was good, but there were many school of thoughts. Coming from python background, there was nose, unittest, mock, py.test etc. Each school thinks they are right. There is however an ambiguity on how we actually define unit in a unit test?

How do you define a unit? Is it the assertion that a response is 500. or is unit all assertions inside a test class. In that case how long should a class be? aprox lines of code?

It will be great if people can share their opinion/experience about it. Appreciate your reading it, thanks!