Confidence, self-confidence, assurance, self-assurance, self-possession, aplomb are comparable when denoting either a state of mind free from diffidence, misgivings, or embarrassment or the easy, cool, or collected bearing or behavior resulting from this attitude.
Confidence stresses faith in oneself and in one’s powers; it does not as a rule imply conceit nor preclude the suggestions of support from external agencies or influences or of modest recognition of that assistance.
When self-sufficiency is connoted, self-confidence commonly replaces confidence.
Assurance is distinguishable from confidence only by its far stronger implication of certainty and its frequent suggestion of arrogance; thus, one meets a situation with confidence when one’s belief in one’s powers is strong, but with assurance when one never questions the outcome or the Tightness of what one is saying or doing.
Self-assurance implies an assured self-confidence.
Self-possession implies an ease or coolness arising from command over one’s powers; it connotes, usually, controlled but not repressed emotions and actions, or speech free from flurry and appropriate to the situation.
Aplomb describes the behavior or, less often, the bearing of one whose assurance or self-possession is conspicuously but not necessarily disagreeably evident.