Caring for Your Introvert gives an insightful definition of introversion, the best I’ve heard:
Introverts are not necessarily shy. Shy people are anxious or frightened or self-excoriating in social settings; introverts generally are not. Introverts are also not misanthropic, though some of us do go along with Sartre as far as to say “Hell is other people at breakfast.” Rather, introverts are people who find other people tiring.
Introverts are people who find other people tiring.