Serenity is a related term of imperturbability.
As a proper noun serenity
is in general use since the 1990s.
As a noun imperturbability is
a state of calm, unruffled self-assurance; aplomb, composure.
serenity
English
Noun
(serenities)
The state of being serene; calmness; peacefulness.
A lack of agitation or disturbance.
A title given to a reigning prince or similar dignitary.
Synonyms
* sereneness
* tranquility / tranquillity
* harmony
* peace
; title
* Serene Highness
See also
*
imperturbability
English
Noun
(-)
A state of calm, unruffled self-assurance; aplomb, composure.
Related terms
* imperturbable