introvert

Concise Oxford English Dictionary © 2008 Oxford University Press:
introvert/ˈɪntrəvɜːt/

▶noun a shy, reticent person.

■ Psychology a person predominantly concerned with their own thoughts and feelings rather than with external things.

Compare with extrovert. ▶adjective of, denoting, or typical of an introvert.

– derivatives
introversion noun,
introversive /-ˈvəːsɪv/ adjective.

– origin C17: from mod. L. introvertere, from intro- ‘to the inside’ + vertere ‘to turn’.