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Synonyms

flu

American  
[floo] / flu /

noun

  1. influenza.

  2. a specific variety of influenza, usually named for its point of dissemination or its animal vector.

    Hong Kong flu; swine flu.
flu British  
/ fluː /

noun

  1. short for influenza

  2. any of various viral infections, esp a respiratory or intestinal infection

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
flu Scientific  
/ flo̅o̅ /

  1. See influenza

Etymology

Origin of flu

First recorded in 1830–40; shortened form

Compare meaning

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Explanation

The flu is a disease that's easily spread between people. When you have body aches, a fever, and a sore throat, you probably have the flu. The word flu is shorthand for "influenza," the official name of this infectious, viral illness. It's not unusual to get the flu, especially in the winter, and while it usually isn't serious, it can be deadly for some groups of people. Your chances of getting the flu are greatly reduced if you get a flu shot. The Italian root influenza means "influenza" or "epidemic," but also "visitation" or "influence of the stars."

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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.

Flu pandemics are inevitable, even if the timing of the next global outbreak is uncertain.

From BBC • Apr. 21, 2026

The following day, she tested positive for Flu A.

From The Wall Street Journal • Mar. 29, 2026

It is not yet clear whether the new H3N2 Flu A subclade K subvariant will reduce the efficacy of the flu vaccine this season.

From Los Angeles Times • Feb. 27, 2026

Flu viruses can change in two different ways.

From Salon • Jan. 8, 2026

That stands upon the rolling restless stone,—7 Flu.

From King Henry the Fifth Arranged for Representation at the Princess's Theatre by Kean, Charles John

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.