tranquillity

noun

1. An absence of motion or disturbance: calm, calmness, hush, lull, peace, peacefulness, placidity, placidness, quiet, quietness, serenity, stillness, untroubledness. See calm/agitation.
2. Lack of emotional agitation: calm, calmness, peace, peacefulness, placidity, placidness, quietude, serenity. See calm/agitation.

Definition: peace, quiet
Antonyms: chaos, disturbance, loudness, noise, turbulence, violence, wildness

The noun has 2 meanings:

Meaning #1: a disposition free from stress or emotion
  Synonyms: repose, quiet, placidity, serenity, tranquillity

Meaning #2: a state of peace and quiet
  Synonyms: tranquillity, quietness, quietude

Quotes:

"The more tranquil a man becomes, the greater is his success, his influence, his power for good. Calmness of mind is one of the beautiful jewels of wisdom." - James Allen

"Like water, we are truest to our nature in repose." - Cyril Connolly

"You must learn to be still in the midst of activity and to be vibrantly alive in repose." - Indira Gandhi

"There is no such thing as perpetual tranquillity of mind while we live here; because life itself is but motion, and can never be without desire, nor without fear, no more than without sense." - Thomas Hobbes

"Tranquility is the old man's milk." - Thomas Jefferson