How does Atticus Finch define what a man is in Chapters 10-11 of To Kill a Mockingbird?
Part of the definition of being a man includes being a good father, a contributing member of society, and a moral citizen.
Atticus does this physically through protecting his children from the dog.
He demonstrates good fathering emotionally and morally through making the tough-love decision of forcing Jem to work things out with Mrs. Dubose. This in turn was a lesson that modeled true courage. After the kids saw her example and experienced her circumstances, Atticus took the time to discuss the definition of courage and explain the difference between the desire to do cool, risky, dangerous things and living through something difficult and making the right choices. This foreshadows the eventual trial wherein we have the opportunity once again to look at the quality of men: Tom Robinson and Bob Ewell. Atticus' parenting and teaching of the children gives them clear understanding between the moral and immoral man by the time they listen to these mens' testimonies.
Atticus' quotes at the end of 11 are worth using to defend any points you intend to make if you are writing an answer to a question for your teacher.
I think that, in these chapters, Atticus Finch defines masculinity in a way that is not very conventional. His kids want him to be more conventionally masculine. They want him to approve of shooting things and they want him to be important.
But I think Atticus is defining masculinity more in the sense of being true to who you are and not giving up. We see that he is really able to shoot well, but does not want to because take unfair advantage (as Calpurnia says). Being a man, to him, is about working hard and doing what you think is right.