WordNet sense 1 (a period of calm weather):

lull

WordNet sense 2 (an untroubled state; free from disturbances):

tranquility, tranquillity

WordNet sense 3 (the absence of sound):

silence

WordNet sense 4 (a disposition free from stress or emotion):

placidity, repose, serenity

WordNet sense 5 (become quiet or quieter):

hush, quiesce, quieten, pipe down, quiet down

WordNet sense 6 (marked by absence of sound):

silent, soundless, still

WordNet sense 7 (not showy or obtrusive):

restrained

WordNet sense 8 (not ostentatious):

unpretentious, unostentatious, unpretending

WordNet sense 9 (marked by a state of tranquil repose):

quiescent

WordNet sense 10 (make calm or still):

tranquilize, tranquillize, tranquillise, calm down

WordNet sense 11 (in a softened tone):

hushed, muted, subdued

WordNet sense 12 ((of sound) relatively low in volume):

soft

WordNet sense 13 ((of a body of water) free from disturbance by heavy waves):

placid, smooth, tranquil, unruffled

WordNet sense 14 ((of weather) free from storm or wind):

calm

WordNet sense 15 (with little or no activity or no agitation (`quiet' is a nonstandard variant for `quietly')):

quietly