: a common disease that is caused by a virus and that causes fever, weakness, body aches, and breathing problems : influenza
He came down with a bad bout/attack/case of the flu.
catch/get the flu
She has the flu. = (Brit) She has flu.
—often used before another nounthe flu season
a flu shot
a flu epidemic
Cite this Entry
“Flu.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/flu. Accessed 18 Apr. 2026.
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