megalopolis; megacity; supercity
Refers to an extremely large urban area, often a conurbation or a city with a population over 10 million. Used in urban planning and geography contexts.
Tokyo is one of the world's leading megacities.
Traffic congestion is a serious problem in megacities.
大都市 is a general term for a large city, while 巨大都市 specifically emphasizes an exceptionally vast scale, often used for megacities or megalopolises.
メガシティ is a direct loanword from English 'megacity' and is used similarly, but 巨大都市 is the native Japanese term and may sound more formal or technical.
Compound of 巨大 (きょだい, 'huge') and 都市 (とし, 'city'). The term is a straightforward modern coinage reflecting the concept of a very large urban area.