Confidence is knowing the value of your knowledge while maintaining an awareness of how much you have left to learn.
Confidence is being willing to defend your beliefs, but also to change them based on new information.
Confidence is sharing what you have to offer without apology and accepting what others have to offer in the same way.
Confidence is understanding that both you and someone else can be right despite your holding different opinions.
Confidence is never needing to prove yourself to anyone but yourself, because you hold yourself to higher standards than anyone else ever could.
Confidence is enjoying what you have, but always striving for more.
Confidence is not gained by winning, nor is it lost by losing.
Confidence is succeeding by your own standards while respecting others’ right to do the same.
Confidence is helping others when and where you can and being willing to accept the help others offer you.
Confidence is worth having, but requires constant maintenance.
Confidence is a good start to a good life.
Update: February 20, 2017
I would add that confidence typically doesn’t require one to peacock and stomp around, belittling others to feel big. A lot of Western cultures in particular seem to conflate arrogance and confidence, and self-conscious acts and confidence, when in fact these things couldn’t be more different.