I was wondering if I could use any of these questions to ask for a definition to another person: "Do you know what a dog is?", "Do you know what is a dog?", "Do you know the meaning of the word dog?"
It sounds correct. Sort of:
A: Dude, I just ran into a dog.
B: Yeah, so what?
A: It had a smokestack and wheels.
B: Dude, do you know what a dog is?
Your first and third examples are correct. The second gets the point across, but it is clumsy.
I know you're just using "dog" as an example here, but it makes your sentence sound rhetorical.