To "fit in" refers to the desire or attempt to conform or belong to a particular group or social setting, often by adopting certain behaviors, attitudes, or appearances.
Unspoken rules that guide behavior within a particular social group. For example, if smoking is considered cool among one's peers, it becomes a social norm.
The influence exerted by one's peers to conform to their beliefs, attitudes, and behaviors. Smoking might be seen as an act of conformity due to peer pressure.
The process through which individuals develop their sense of self and establish their place within various social groups. Smoking could be viewed as an attempt for teenagers to shape their identity and find acceptance among certain peer groups.