This is a good question. As Scout learns about the trial, she naturally asks what rape is, as Tom Robinson is on trial because of an alleged rape. So, at first Scout asks Calpurnia. Calpurnia feels uncomfortable in giving an answer, so she tells Scout to ask her father, Atticus. When Atticus finally comes home, Scout asks. Here is the dialogue:
“What’s rape?” I asked him that night. Atticus looked around from behind his paper. He was in his chair by the window. As we grew older, Jem and I thought it generous to allow Atticus thirty minutes to himself after supper.
He sighed, and said rape was carnal knowledge of a female by force and without consent.
“Well if that’s all it is why did Calpurnia dry me up when I asked her what it was?”
As you can see Atticus was not able to deflect the question. So, he answered her in a direct manner. However, he spoke is a lawyerly way - carnal knowledge of a female by force without consent.
The funny thing is that Scout acted like she understood this definition, when in fact she had no clue what Atticus was saying. The point is that Atticus gave her an answer, but it was above Scout's head and understanding. In doing so, this answer satisfied Scout (she was given an answer), but also did not explain what rape was, as Scout was too young to understand. In the end, Scout and Atticus were satisfied.