I think the definition of "confidence" is trusting our own judgement in decision making, whether that judgement concerns work decisions or life decisions. This also implies that we aren't afraid to learn from our mistakes if our judgement about a particular matter is wrong, and being grateful for learning something new.
I've found that the most effective way to build self confidence - both professionally or personally - is to create situations in which we succeed and build affirmations around those successes for ourselves.
For example, let's say a sales representative is having trouble closing deals. If they simply start saying to themselves, "I can do this. I'm a great salesperson," their subconscious mind will sabotage them if they don't have self confidence. The solution is to review past failures and successes, determine what worked (or didn't), and research what would work better. Rehearse these scenarios in the mirror, with a trusted friend or loved one, or a coworker. When you get it right, create your affirmation around that success. "I now know what I have to do - I worked hard at it, and nailed it when I practiced!" Remember that success when you pitch the next client, and create another positive affirmation around each success. "I closed Clients X and Y! I know I can do this."
This can be applied to one's personal life, too. I call it "The New Car Factor." When we get a new car, suddenly we see and recognize all the cars just like it that we might not have noticed before. Our new car has become familiar. When we see and recognize all the small successes we have - or have created for ourselves - we now recognize more and more of those little successes and create self confidence through them. They become comfortable and familiar.
We can create little successes for ourselves every day. Holding a door open for someone or letting another driver into traffic is a success in kindness. Helping a coworker with a difficult project when you don't have to is a success in teamwork. Listening to a friend vent about a problem is a success in compassion. By remembering these successes and building affirmations around them, we build both our self esteem and our self confidence.