WordWeb Online

1. (of a body of water) free from disturbance by heavy waves
   "a lake of tranquil blue water reflecting a tranquil blue sky"
   - placid, quiet, still, smooth, unruffled
    
2. Not agitated; without losing self-possession
   "she became more tranquil"; "tranquil life in the country"
   - calm, unagitated, serene

See also: calm, composed, tranquility, tranquillity, tranquilly

Encyclopedia: Tranquil Tranquil, Mississippi