Self-confidence is a characteristic of believing firmly in your own abilities and strengths. It took a lot of self-confidence to speak up in that student council meeting and say what you were thinking!
If you have no doubt that you'll be able to perform your new song at the school talent show, you've got self-confidence. We've been using this noun since the 17th century for a particular type of confidence that's focused on individual strength and the absence of self-doubt. Your self-confidence is what allows you to do things confidently, assuredly, and with conviction.