Tumor vs. tumour

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For the noun referring to an abnormal tissue growth, tumor is the preferred spelling in American English. Tumour is the standard spelling outside North America. Canadians likewise prefer tumour by a large margin, though tumor does appear in Canadian publications with relative frequency.

Tumour is one of numerous -our-ending words that lost the in American English at some point during the 19th century. In this case, the was dropped for good by around 1880, though tumour has never fully disappeared from U.S. writing, and in fact a few medical publishers still prefer it.

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