serious

UK: /ˈsɪəriəs/ US: /ˈsɪriəs/ ,(sēr′ē əs)

WordReference Random House Learner's Dictionary of American English © 2025

se•ri•ous /ˈsɪriəs/   adj. 

1. of or characterized by thought:
   a serious study of inner-city violence.
2. grave, solemn, or somber, as in mood;
   not cheerful:
   He was very serious and didn't even smile.
3. earnest;
   sincere:
   a serious offer to buy the house.
4. requiring thought or concentration:
   serious reading.
5. important or significant:
   Marriage is a serious matter.
6. giving cause for worry or fear:
   a serious relapse.

se•ri•ous•ness, n. [uncountable]

WordReference Random House Unabridged Dictionary of American English © 2025

se•ri•ous  (sēr′ē əs), 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. Medicine (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. 

8. that which is of importance, grave, critical, or somber:
   You have to learn to separate the serious from the frivolous.

• Latin sērius or Late Latin sēriōsus; see -ous, -ose¹
• late Middle English 1400–50

se′ri•ous•ness, n. 
Synonyms

• 2. sober, sedate, staid.
• 3. See earnest ¹.
• 5. momentous, grave.

Antonyms

• 3, 5. trivial.

Collins Concise English Dictionary © HarperCollins Publishers::

serious / ˈsɪərɪəs/ 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. worthy of regard because of substantial quantity or quality: serious money, serious wine
7. extreme or remarkable: a serious haircut

Etymology: 15th Century: from Late Latin sēriōsus, from Latin sērius; probably related to Old English swǣr gloomy, Gothic swers esteemed

ˈseriousness noun