The difference between a Pandemic and an Epidemic

I don't have the source for this; probably something I wrote down from the NYT or NYRB:

Pandemic, an adjective from the Greek pandemos, "of all the people," becomes a noun to mean "the outbreak of a disease spreading over a large geographic area," now construed as "worldwide." Epidemic, disease visited on a large segment of a population, is now considered regional rather than global.