British spelling of vapor; a gaseous form of a substance that is normally liquid or solid.
From Old French vapour, from Latin vapor 'steam, heat.' The 'u' spelling reflects French influence in Middle English, preserved in British English while American English simplified to 'vapor.'
The spelling difference between 'vapour' and 'vapor' tells the story of American linguistic independence - Noah Webster deliberately removed the 'u' to create a distinctly American English, making every instance of this word a tiny declaration of national identity.
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