say it: trang-KWIL-i-tee
tranquillity is a noun meaning "an untroubled state; free from disturbances". It carries 3 recorded senses.
an untroubled state; free from disturbances
a state of peace and quiet
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for tranquillity come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.