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";
   - calm, unagitated, serene
    
3. Free of noise and disturbance, with little activity
   "a tranquil scene by the river";
   - calm, peaceful, serene

See also: calm, composed, halcyon, quiet

Encyclopedia: Tranquil, Mississippi