Dictionary.com
Thesaurus.com
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

How does flu compare to similar and commonly confused words? Explore the most common comparisons:

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."

Keep Reading on Vocabulary.com

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.

Blue flu is when a large number of police officers take sick leave as a form of protest.

From Los Angeles Times • Apr. 24, 2026

It is impossible to know if H5N1 will be the strain that causes the next flu pandemic.

From BBC • Apr. 21, 2026

Abbott Laboratories cut its full-year earnings outlook to account for a recent acquisition and said a weaker-than-expected flu season hurt its recent quarter.

From The Wall Street Journal • Apr. 21, 2026

He might not be sure that the flu vaccine caused his voice disorder, but he’s willing to list it as a culprit.

From Slate • Apr. 18, 2026

Then Reb Saunders became ill, and at the same time my father also took to his bed with the flu, a severe case that bordered on pneumonia for a while and frightened me terribly.

From "The Chosen" by Chaim Potok

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.