[nerdponx]

That's why they said "often".

A better definition might be "the smallest amount of testable functionality".

[exdsq]

Which is a function.

[JoeNr76]

It might start out as a function, then become a factory, then an object and if you bind your test to the implementation details of this code, you will have to rewrite your test every time you refactor your code. Not ideal.