Chelsea Syllabification: (Chel·sea)Pronunciation: /ˈCHelsē/Definition of Chelsea 1an industrial and commercial city in northeastern Massachusetts, just north of Boston; population 41,577 (est. 2008). 2a residential district of London, on the northern bank of the Thames River. 3a fashionable residential section of southern Manhattan in New York City, on the west side of the city. Chelsea in other Oxford dictionaries Definition of Chelsea in the British & World English dictionary