Merriam-Webster Collegiate Dictionary - calm

 I. noun  Etymology: Middle English ~e, probably ultimately from Old Spanish ~a, from Late Latin cauma heat, from Greek kauma, from kaiein to burn  Date: 14th century  1.  a. a period or condition of freedom from storms, high winds, or rough activity of water  b. complete absence of wind or presence of wind having a speed no greater than one mile (1.6 kilometers) per hour — see Beaufort scale table  2. a state of tranquillity  II. verb  Date: 14th century  intransitive verb to become ~ — usually used with down  transitive verb to make ~ — often used with down  III. adjective  Date: 14th century  1. marked by ~ ; still a ~ sea  2. free from agitation, excitement, or disturbance a ~ manner  • ~ly adverb  • ~ness noun Synonyms:  ~, tranquil, serene, placid, peaceful mean quiet and free from disturbance. ~ often implies a contrast with a foregoing or nearby state of agitation or violence the protests ended, and the streets were ~ again. tranquil suggests a very deep quietude or composure the tranquil beauty of a formal garden. serene stresses an unclouded and lofty tranquillity watched the sunset of a serene summer's evening. placid suggests an undisturbed appearance and often implies a degree of complacency remained placid despite the criticism. peaceful implies a state of repose in contrast with or following strife or turmoil grown peaceful in old age.