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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Related Words

See earnest ¹.

Other Word Forms

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

Serious side effects were reported at similar rates in both groups, indicating that the treatment was generally well tolerated.

From Science Daily • Mar. 30, 2026

I wrote a book called “Happiness Is a Serious Problem” in 1998, which stressed two important insights.

From The Wall Street Journal • Feb. 26, 2026

The Grammy-nominated singer-songwriter announced on Instagram Tuesday that they were canceling five shows and pushing back the start of their It’s Not That Serious tour.

From Los Angeles Times • Feb. 10, 2026

Explaining his decision to bring his bitter rival back into the fold as business secretary and a member of the House of Lords, Brown said: "Serious people are needed for serious times."

From BBC • Feb. 3, 2026

She looked like Mom again, with her Serious Business hair and her dirty hands and her crackling eyes, but she wasn’t exactly the same.

From "The Science of Breakable Things" by Tae Keller

Related Words

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