As nouns the difference between tranquility and placidity
is that tranquility is an alternative spelling of lang=en while placidity is the state of being placid; peacefulness.
tranquility
English
Noun
(en-noun)
Usage notes
* More common spelling in the US; less common in the UK.
placidity
English
Noun
(-)
The state of being placid; peacefulness.
Synonyms
* (state of being placid) composure, peacefulness, serenity
Related terms
* placid
* placidly
* placidness