Synonyms: unflappable, imperturbable
Definition: not easily perturbed or excited or upset; marked by extreme calm and composure
Usage: hitherto imperturbable, he now showed signs of alarm; an imperturbable self-possession; unflappable in a crisis
Similar words: composed
Definition: serenely self-possessed and free from agitation especially in times of stress
Usage: the performer seemed completely composed as she stepped onto the stage; I felt calm and more composed than I had in a long time