Pronunciation: (sēr'ē-us),[key] —adj. 1. of, showing, or characterized by deep thought.
2. of grave or somber disposition, character, or manner: a serious occasion; a serious man. 3. being in earnest; sincere; not trifling: His interest was serious. 4. requiring thought, concentration, or application: serious reading; a serious task. 5. weighty or important: a serious book; Marriage is a serious matter. 6. giving cause for apprehension; critical: The plan has one serious flaw. 7. Med.(of a patient's condition) having unstable or otherwise abnormal vital signs and other unfavorable indicators, as loss of appetite and poor mobility: patient is acutely ill.
—n. that which is of importance, grave, critical, or somber: You have to learn to separate the serious from the frivolous.