1.Definition: (of a body of water) free from disturbance by heavy waves

"A lake of tranquil blue water reflecting a tranquil blue sky"

Related Adjective(s):placid, quiet, smooth, still, unruffled

2.Definition: not agitated; without losing self-possession

"She became more tranquil", "Tranquil life in the country"

Related Adjective(s):calm, serene, unagitated