Most definitions of confidence describe a feeling. That is the reason they are not useful: feelings move, and a confidence that moves with them is not unshakeable by definition. A more workable description is structural — confidence is the accumulated evidence that you can rely on yourself.

Psychologists studying this tend to separate two things that everyday language jams together. Self-efficacy, the term Albert Bandura introduced, is your belief that you can execute a specific task. Self-esteem is your global evaluation of your own worth. Unshakeable confidence sits closer to the first: it is domain-specific, evidence-based, and therefore something you can deliberately build rather than something you have to feel your way into.

The word unshakeable is doing real work here. It does not promise the absence of pressure. It describes what happens under pressure: the structure holds. A rejection, a bad quarter, or a room full of people who disagree with you registers as information rather than as a verdict on who you are.