In chapter 16, of Harper Lee's novel To Kill a Mockingbird, Atticus is very specific about his definition of what constitutes a mob.
A mob’s always made up of people, no matter what. Mr. Cunningham was part of a mob last night, but he was still a man. Every mob in every little Southern town is always made up of people you know—doesn’t say much for them, does it?”
Earlier in the chapter, Atticus states that mobs do not exist in Maycomb.
“No, we don’t have mobs and that nonsense in Maycomb. I’ve never heard of a gang in Maycomb.”
Atticus, therefore, bases his definition of a mob off of one particular characteristic: "being made up of people." Therefore, Atticus is simply stating that only one thing universalizes a mob, it is simply a group of people.
Atticus does this so as to calm the fears of his children, Jem and Scout. Both are worried about the repercussions Atticus is going to face given his taking on of Tom Robinson's rape case. Both Jem and Scout are worried about backlash which they believe Atticus is going to face and Atticus calms their fears by marginalizing what a mob truly is.
Following the incident involving the Old Sarum bunch outside of Tom Robinson's cell, Atticus explains to his children mob mentality. In Chapter 16, Atticus tells Jem and Scout that "every mob in every little Southern town is made up of people you know---doesn't say much for them, does it?" (Lee 97). Atticus realizes that a mob is made up of individuals with similar views. He also refers to them as a "gang of wild animals" who are still human. When a group of unruly men gets together, they have a tendency to act like a mob and lose their individuality. Mr. Cunningham is a good example of a morally upright person who succumbs to mob mentality. In a mob, people are influenced by their peers and adopt certain behaviors. Each person loses their individuality in a mob by embracing the group's attitude and perspective. Atticus essentially defines a mob as being a group of unruly individuals who adopt each other's negative mentality and act like wild animals.