serious

American  

[seer-ee-uhs] / ˈsɪər i əs /

adjective

1. of, showing, or characterized by deep thought.

2. of grave or somber disposition, character, or manner.

   a serious occasion; a serious man.

   Synonyms:

   staid, sedate, sober

3. being in earnest; sincere; not trifling.

   His interest was serious.

   Antonyms:

   trifling, frivolous

4. requiring thought, concentration, or application.

   serious reading; a serious task.

5. weighty or important.

   a serious book; Marriage is a serious matter.

   Synonyms:

   grave, momentous

   Antonyms:

   trivial

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.

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noun

1. that which is of importance, grave, critical, or somber.

   You have to learn to separate the serious from the frivolous.

serious British  

/ 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. informal worthy of regard because of substantial quantity or quality

   serious money

   serious wine

7. informal extreme or remarkable

   a serious haircut

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Synonym Usage

See earnest ¹.

Other Word Forms

Derived Forms

• overserious
  adjective
• seriousness
  noun
• unserious
  adjective
• quasi-serious
  adjective
• unseriously
  adverb
• half-serious
  adjective
• half-seriously
  adverb
• quasi-seriously
  adverb
• nonserious
  adjective
• overseriously
  adverb
• ultraserious
  adjective
• superseriously
  adverb
• ultraseriously
  adverb
• superserious
  adjective
• nonseriously
  adverb

Etymology

Origin of serious

First recorded in 1400–50; late Middle English, from Middle French serieux and Latin sērius “grave, earnest” or Late Latin sēriōsus; see -ous, -ose ¹

Explanation

Any important situation is serious, and this word is also the opposite of humorous and playful. If you're frowning, someone might say, "Why so serious?" This is a word for people and situations where there's no fooling around. If you're staring at the computer screen, typing quickly, you look serious. If you are in a car accident, that's a serious situation. Some people are more serious than others. If you're always making jokes and smiling, then you're not that serious. One place everyone needs to be serious is at a funeral home.

Example Sentences

“This initiative reflects the serious crisis we face and that affordability is a real thing,” said Vikas Saini, president of the Lown Institute, a Massachusetts-based healthcare think tank.

From Los Angeles Times • Jun. 10, 2026

It can’t be all of them, because there’s some pretty serious theological differences that invariably bleed into politics.

From Salon • Jun. 10, 2026

Full-back Malacia chalked up 50 appearances between 2022 and 2026 but saw his stint at the club disrupted by a serious knee injury.

From BBC • Jun. 10, 2026

New research in mice suggests that gut microbes and the compounds they produce may play a surprising role in protecting against some of the condition's most serious consequences, including heart disease.

From Science Daily • Jun. 9, 2026

I look at him, try to judge whether or not he’s serious.

From "Red Flags and Butterflies" by Sheryl Azzam

Related Words

• dangerous
• deep
• difficult
• far-reaching
• grievous
• important
• major
• meaningful
• severe
• significant
• tough
• urgent