There's more than one place called '''Jefferson County''':
Jefferson County (Alabama) - in the Metropolitan (Alabama) region of Alabama
Jefferson County (Arkansas)
Jefferson County (Colorado) - in the Front Range region of Colorado
Jefferson County (Florida)
Jefferson County (Georgia)
Jefferson County (Idaho)
Jefferson County (Illinois)
Jefferson County (Indiana)
Jefferson County (Iowa)
Jefferson County (Kansas)
Jefferson County (Kentucky)
Jefferson Parish (Louisiana)
Jefferson County (Mississippi)
Jefferson County (Missouri)
Jefferson County (Montana)
Jefferson County (Nebraska) - in Eastern Nebraska
Jefferson County (New York)
Jefferson County (Ohio)
Jefferson County (Oklahoma)
Jefferson County (Oregon)
Jefferson County (Pennsylvania)
Jefferson County (Tennessee)
Jefferson County (Texas)
Jefferson County (Washington) - in the Olympic Peninsula region of Washington (state)
Jefferson County (West Virginia)
Jefferson County (Wisconsin)
See also:
Jefferson Davis Parish - in Louisiana
Jefferson Davis County - in Mississippi