Serenity - 1. serenity - a disposition free from stress or emotion tranquility, placidity, tranquillity, repose, quiet calm, calmness, composure, equanimity - steadiness of mind under stress; 'he accepted their problems with composure and she with equanimity' ataraxia - peace of mind 2. serenity - the absence of mental stress or anxiety ataraxis, peace of mind, peacefulness, repose, peace, heartsease quietude, quietness, tranquillity, tranquility - a state of peace and quiet