Define the concept of mediator and moderator

In the research scenario below, define the mediator and moderator / independent and dependent variable. Include evidence to support the definitions of the mediator and moderator for the research situation.

Scenario:

Being socially excluded leads to increases in aggressive behavior. Research has found that when people are excluded by others, they are more likely to behave aggressively, even to people who did not initially exclude them (Twenge et al., 2001).