Definition: With regard to relationships and human behavior, social exclusion refers to the act of rejecting someone from interpersonal interactions. Social exclusion may or may not be intentionally harmful.
When it is intentional, social exclusion is considered to be a form of relational aggression or social aggression. Intentionally harmful social exclusion may be overt, such as not talking to an individual, or it may be more subtle, such as by spreading rumors about a person so that she gradually becomes rejected.
Social exclusion is most often performed by girls, especially when they're threatened with being rejected themselves. Boys do, however, also engage in intentional social exclusion.
Archer, John, and Coyne, Sarah. An integrative review of indirect, social, and relational aggression. 2005. Personality and Social Psychology Review. 9, 3: 212-230.
Benenson, Joyce F., Markovits, Henry, Thompson, Melissa Emery, and Wrangham, Richard W. Under threat of social exclusion, females exclude more than males. 2011. Psychological Science.