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

Food allergies can cause mild or very serious reactions, which is different to a food intolerance.

From BBC • Apr. 23, 2026

Panama's foreign ministry confirmed the seizure of the MSC Francesca, calling it a "serious attack on maritime security" and an "unnecessary escalation".

From Barron's • Apr. 23, 2026

He also saw “pre-project metadata, hidden defined names, institutional data-terminal markers, real lender or counterparty names, irregular numeric precision, and other features that raised serious provenance questions,” the suit said.

From The Wall Street Journal • Apr. 23, 2026

We already know how to govern legal, profitable activities that carry serious health risks.

From Los Angeles Times • Apr. 23, 2026

Grandpa got that serious look in his eyes.

From "Summer of the Monkeys" by Wilson Rawls

Related Words

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