To officially declare someone a saint (British spelling), or to admit into the canon of recognized saints.
From canon plus -ise suffix (British spelling of -ize). The meaning evolved from 'to include in a list of authorized writings' to 'to officially recognize as a saint.'
The British spelling 'canonise' is the same process as the American 'canonize'—when the Church officially says someone lived such a holy life they deserve to be a saint forever.
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