Overspill, in this sense, is people who leave a big city because too many people already live there, and go to new houses outside the city. Planners also use it for the new towns and housing built for those people: overspill towns, overspill housing. It is mainly a British word, from city planning in the last century — London overspill is a famous example. The picture behind it is a cup that is too full, so the drink runs over the edge; here the "cup" is the crowded city, and the "spill" is people. You also hear it as a verb: a city overspills into the towns around it. It is not a word for water on the floor; that is just spill. In the US people more often say overflow, or talk about moving out to the suburbs. Though actually, even in Britain younger speakers may not use overspill much unless they are talking about housing history or planning. The word can sound a little cold, as if people were liquid. That is the tidy planning word; the people themselves just say they moved.

人口外溢（都市計畫用語；不是打翻液體）