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
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.
It’s a playbook, which involves more than slashing head count, that he used to pull British retailer Tesco out of a serious slump.
From The Wall Street Journal ● Aug. 7, 2026
So far, many patients can receive large doses several times a week without serious side-effects.
From Science Daily ● Aug. 7, 2026
He said that because the "death of Andrew was such a serious and tragic case", Bowers and Coles should "not be afforded the opportunity" to be released early.
From BBC ● Aug. 7, 2026
But as with many vaccines, more serious complications are extremely rare.
From Slate ● Aug. 6, 2026
Grandpa seemed to be so serious in what he was saying, it was funny.
From "Summer of the Monkeys" by Wilson Rawls
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.
