aplomb

–noun
1. imperturbable self-possession, poise, or assurance.
2. the perpendicular, or vertical, position.
[Origin: 1820–30; < F à plomb according to the plummet, i.e., straight up and down, vertical position]

—Synonyms 1. composure, equanimity, imperturbability.
—Antonyms 1. confusion, discomposure; doubt, uncertainty.