adjective: 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. Medicine/Medical. (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. noun: 8. that which is of importance, grave, critical, or somber: You have to learn to separate the serious from the frivolous. Related words: genuine, deliberate, sincere, thoughtful, honest, severe, major, grievous, dangerous, deep, far-reaching, urgent, important, significant, tough, meaningful, difficult, sober, austere, bound. Origin: 1400–50; late Middle English < Latin sērius or Late Latin sēriōsus; see -ous, -ose1. Synonyms: 2 sober, sedate, staid. 3 See earnest1. 5 momentous, grave. Antonym: 3, 5 trivial. British dictionary: adjective: 1. grave in nature or disposition; thoughtful: a serious person. 2. marked by deep feeling; in earnest; sincere: is he serious or joking? 3. concerned with important matters: a serious conversation. 4. requiring effort or concentration: a serious book. 5. giving rise to fear or anxiety; critical: a serious illness. 6. Informal. worthy of regard because of substantial quantity or quality: serious money; serious wine. 7. informal extreme or remarkable: a serious haircut. Origin: C15: from Late Latin sēriōsus, from Latin sērius; probably related to Old English swǣr gloomy, Gothic swers esteemed. Medicine definition: adjective: 1. Being of such import as to cause anxiety, as of a physical condition. Source.