Introversion
This article deals with the psychological term Introversion. If you are looking for the games developer, see Introversion Software.
The terms Introvert and Extrovert (spelled Extravert by Carl Jung), were originally employed by Sigmund Freud and given significant amplification later by Jung. The terms refer to "attitudes" and show how a person orients and receives their energy. In the Extraverted attitude the energy flow is outward, and the preferred focus is on people and things, whereas in the Introverted attitude the energy flow is inward, and the preferred focus is on thoughts and ideas.
Thus, one who is introverted is more likely to spend time alone or in contemplation, as these activities are rewarding. They may avoid social situations entirely, not because of shyness, but because they choose to.