Any massive, bustling city with a globally diverse population might be described as a cosmopolis. New York City, with its millions of people, immense political and financial influence, and melting pot of cultures, is the quintessential modern cosmopolis.
Cosmopolis stems from two Greek words: cosmos, meaning "world," and polis, meaning "city." Literally a "world-city," a cosmopolis is a place that feels like a universe unto itself. Because it's a hub of international influence, it acts a bit like a microcosm of the globe — a single city where people from almost every background, culture, and country intersect. The term cosmopolitan, "composed of or familiar with people from all over the world," is an adjective form of the same concept.