Calm, tranquil, serene, placid, peaceful, halcyon mean quiet and free from all that disturbs or excites.
Calm is primarily applied to sea or weather, usually conveys an implicit contrast with its opposite, stormy, and suggests freedom, real or assumed, from agitation of whatever sort.
Tranquil implies a more settled composure, a more inherent quiet, than calm with less suggestion of previous agitation overcome.
Serene suggests a lofty and unclouded tranquillity.
Placid connotes lack of excitement and suggests an unruffled and equable aspect or temper or even sometimes, in derogatory use, a hint of stupidity.
Peaceful (see also PACIFIC) implies repose or the attainment of undisturbed tranquillity.
Halcyon implies an almost magic or golden calmness especially of weather or of spirit.