Which word represents complete and confident composure or self-assurance?

The word "aplomb" specifically denotes a state of complete and confident composure or self-assurance in both challenging and routine situations. It suggests a level of poise and assurance that allows an individual to handle situations with grace and effectiveness. This term is frequently used in contexts where one not only maintains calmness under pressure but does so with a sense of style or elegance, indicating a high degree of confidence.

While "calmness" refers to a peaceful state lacking agitation, and "composure" indicates a balance of emotions and reactions, "aplomb" encompasses a broader, more dynamic range of self-assurance. "Assurance" relates more to a guarantee or confidence about a specific outcome than to the overall presence of confidence in one’s demeanor. Thus, "aplomb" is the most fitting choice for representing both complete composure and self-assurance in an eloquent manner.