Ambivert

Ambiversion
Although many people view being introverted or extraverted as a question with only two possible answers, most contemporary trait theories measure levels of extraversion-introversion as part of a single, continuous dimension of personality, with some scores near one end, and others near the half-way mark,[10] see the Big Five personality traits. Ambiversion is falling more or less directly in the middle.[4][11] An ambivert is moderately comfortable with groups and social interaction, but also relishes time alone, away from a crowd.

http://en.wikipedia.org/wiki/Ambiverts#Ambiversion