serious
Americanadjective
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of, showing, or characterized by deep thought.
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of grave or somber disposition, character, or manner.
a serious occasion; a serious man.
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being in earnest; sincere; not trifling.
His interest was serious.
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requiring thought, concentration, or application.
serious reading; a serious task.
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weighty or important.
a serious book; Marriage is a serious matter.
- Antonyms:
- trivial
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giving cause for apprehension; critical.
The plan has one serious flaw.
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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
adjective
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grave in nature or disposition; thoughtful
a serious person
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marked by deep feeling; in earnest; sincere
is he serious or joking?
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concerned with important matters
a serious conversation
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requiring effort or concentration
a serious book
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giving rise to fear or anxiety; critical
a serious illness
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informal worthy of regard because of substantial quantity or quality
serious money
serious wine
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informal extreme or remarkable
a serious haircut
Synonym Usage
See earnest 1.
Other Word Forms
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seriousnessnoun
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half-seriousadjective
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nonseriousadjective
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overseriousadjective
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quasi-seriousadjective
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superseriousadjective
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ultraseriousadjective
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unseriousadjective
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half-seriouslyadverb
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nonseriouslyadverb
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overseriouslyadverb
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quasi-seriouslyadverb
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superseriouslyadverb
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ultraseriouslyadverb
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unseriouslyadverb
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.
Vocabulary lists containing serious
The SAT: Words to Capture Tone, List 3
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The SAT: Multiple-Meaning Words, List 5
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The New SAT: Words to Capture Tone
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Too often, soccer institutions seem willing to tolerate serious allegations when the player in question remains valuable on the field.
From Salon ● Jul. 18, 2026
"The Australian Government is deeply frustrated and bitterly disappointed that authorities in Laos are not pursuing the most serious charges," Foreign Minister Penny Wong said on Friday.
From Barron's ● Jul. 18, 2026
IBM has spent decades working on the idea of quantum computing, getting increasingly more serious in the past 10 years.
From The Wall Street Journal ● Jul. 18, 2026
Flintoff suffered serious facial and rib injuries when the open-topped three-wheeled Morgan Super 3 he was driving rolled over.
From BBC ● Jul. 17, 2026
I made my brow furrow and my eyes big just like hers so she’d know I was serious, too.
From "Ida B" by Katherine Hannigan
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.