When I am quizzed on a definition, I think of the definition and press "Show." The "Got it" grade is selected by default. This is probably correct most of the time, but I'd actually prefer a lower grade as the default, just to help keep me honest (because they are so quick, definitions are where I'm probably the least careful in grading myself).

Would it be possible to have a setting to toggle the default grade on definitions?

I don't think so--this would add a lot of overhead to the definition prompts, as most definition prompts are answered correctly (like you mention).

One thing you could do is press the "correct" button instead of the "show" button (or its keyboard shortcut, "V"), which shows the answer and marks you wrong. That would do what you want, I think!

and if I recall well, pressing Enter indicates that we know the defintion/writing/reading and grades it as 3 by default, right?

Right--the border and correct button start off in the correct state for all prompts, so just skipping past something gets it out of your way the same as if you'd done it.