Synonyms for Tranquillity (same or very similar meaning)
WordNet sense 1 (an untroubled state; free from disturbances):
quiet, tranquility
WordNet sense 2 (a state of peace and quiet):
quietness, quietude
WordNet sense 3 (a disposition free from stress or emotion):
placidity, repose, serenity
From the ODE community, based on WordNet
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