the GNU version of the Collaborative International Dictionary of English
adj. Incapable of being disturbed or disconcerted.
The Century Dictionary and Cyclopedia
Incapable of being perturbed or agitated; unmoved; self-contained; calm.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. not easily perturbed or excited or upset; marked by extreme calm and composure
Word Usage
"And the other, calm-eyed, cool-browed, serene; strong in her own integrity, with faith in herself, thoroughly at ease; dispassionate, imperturbable; a figure chiselled from some cold marble quarry."