1. Feeling of uncertainty about one’s abilities, actions, etc.
  2. (verb) To call into question the truth of. To be uncertain or in doubt about [something or someone].
  3. A lack of confidence.
  4. An inclination not to believe or accept.
  5. Uncertainty of belief or opinion that often interferes with decision-making.
  6. A state of affairs giving rise to uncertainty, hesitation, or suspense.

The emotion of self-doubt is found in the Heart meridian in the Fire element.

Definition from Dictionary by Merriam-Webster.