WordNet sense 1 (steadiness of mind under stress):

calmness, composure, equanimity

WordNet sense 2 (make calm or still):

lull, tranquilize, tranquillize, quieten, tranquillise, calm down

WordNet sense 3 (become quiet or calm, especially after a state of agitation):

cool off, chill out, cool it, simmer down, settle down

WordNet sense 4 (living without undue worry; taking life easy):

easygoing

WordNet sense 5 (satisfied or showing satisfaction with things as they are):

content, contented, content

WordNet sense 6 (not agitated; without losing self-possession):

serene, unagitated

WordNet sense 7 (serenely self-possessed and free from agitation especially in times of stress):

composed

WordNet sense 8 (make steady):

becalm, steady

WordNet sense 9 ((of a body of water) free from disturbance by heavy waves):

placid, quiet, smooth, still, tranquil, unruffled

WordNet sense 10 (cause to be calm or quiet as by administering a sedative to):

sedate