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
Derived Forms
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unseriousadjective
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half-seriousadjective
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unseriouslyadverb
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quasi-seriousadjective
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seriousnessnoun
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overseriousadjective
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superseriouslyadverb
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ultraseriousadjective
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nonseriouslyadverb
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superseriousadjective
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ultraseriouslyadverb
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half-seriouslyadverb
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overseriouslyadverb
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quasi-seriouslyadverb
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nonseriousadjective
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
Want to remember this word for good? Start your learning journey today with our library of interactive, themed word lists built by the experts at Vocabulary.com – we'll help you make the most of your study time!
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.
But in some cases of severe infection, the immune system can overcorrect and send out an excess amount of these proteins, a serious and potentially fatal reaction called a cytokine storm.
From Los Angeles Times • Jun. 15, 2026
“Not only will you likely be disqualified from receiving SSI benefits, but you could face fines, repayment of benefits, and other serious repercussions.”
From MarketWatch • Jun. 15, 2026
Clayton may be in some respects a more serious choice, but the law creating the office back in 2004 required that the director have “extensive national security expertise”—something Clayton lacks as well.
From Slate • Jun. 15, 2026
The Japanese trio come on stage with serious, even imposing, expressions, but it was not long before they couldn't help themselves – first with a few smirks and then breaking out into infectious grins.
From BBC • Jun. 15, 2026
Sam decided, as he was putting on his jacket for the playground, that when he got home he would have a serious talk with his mom and daddy and Anastasia, too, about safety and health.
From "All About Sam" by Lois Lowry
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.
