As nouns the difference between tranquility and tranquilness
is that tranquility is an alternative spelling of lang=en while tranquilness is the state or quality of being tranquil; tranquillity.
tranquility
English
Noun
(en-noun)
Usage notes
* More common spelling in the US; less common in the UK.
tranquilness
English
Noun
(-)
The state or quality of being tranquil; tranquillity.