imperturbability

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.

Philosophy is a synonym of imperturbability.

As nouns the difference between philosophy and imperturbability

is that philosophy is (uncountable|originally) the love of wisdom while imperturbability is a state of calm, unruffled self-assurance; aplomb, composure.

As a verb philosophy

is to philosophize.