'tranquillity' originates from Latin, specifically the word 'tranquillus' and the noun-forming suffix '-itas' (Late Latin 'tranquillitas'), where 'tranquillus' meant 'quiet, calm'.

'tranquillity' changed from Late Latin 'tranquillitas' and Old French 'tranquilité' into Middle English 'tranquillite' and eventually became the modern English 'tranquillity'.

Initially, it meant 'calmness, quietness', but over time it evolved into its current usage covering both external quiet and internal mental calm ('peaceful state').