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1660s, from Late Latin pandemus, from Greek pandemos "pertaining to all people; public, common," from pan- "all" (see pan-) + demos "people" (see demotic). Modeled on epidemic. The noun is first recorded 1853, from the adjective.
pandemic pan·dem·ic (pān-děm'ĭk)adj. Epidemic over a wide geographic area. n. A pandemic disease.
A widespread epidemic affecting a large part of the population.