Which term could be defined as the treatment usually unfair of a person because of the perception that he or she belongs to a certain social group?

The term you are looking for is "discrimination." It refers to the unfair treatment of individuals based on characteristics such as race, gender, age, or other social group affiliations. This treatment often stems from stereotypes and biases, leading to unequal opportunities and rights for those targeted. Discrimination can occur in various settings, including workplaces, schools, and public spaces.