Shyness is the feeling of apprehension, lack of comfort, or awkwardness especially when a person is around other people. This commonly occurs in new situations or with unfamiliar people; a shy person may simply opt to avoid these situations. Although shyness can be a characteristic of people who have low self-esteem, the primary defining characteristic of shyness is a fear of what other people will think of a person's behavior. This fear of negative reactions such as being mocked, humiliated or patronized, criticized or rejected can cause a shy person to retreat. Stronger forms of shyness can be referred to as social anxiety or social phobia.

Extraversion and introversion are a central trait dimension in human personality theory. The terms were introduced into psychology by Carl Jung, though both the popular understanding and current psychological usage are not the same as Jung's original concept. Extraversion tends to be manifested in outgoing, talkative, energetic behavior, whereas introversion is manifested in more reflective and reserved behavior. Jung defined introversion as an "attitude-type characterised by orientation in life through subjective psychic contents", and extraversion as "an attitude-type characterised by concentration of interest on the external object".