WordWeb Online

1. Dignified and sombre in manner or character and committed to keeping promises
   "a solemn promise"; "the judge was solemn as he pronounced sentence";
   - grave, sedate, sober
    
2. Characterized by a firm and humourless belief in the validity of your opinions
   "a film with a solemn social message";
   - earnest, sincere

Derived forms: solemner, solemnest

See also: serious

Encyclopedia: Solemn