1773 (as St. A Claus, in "New York Gazette"), American English, from dialectal Dutch Sante Klaas, from Middle Dutch Sinter Niklaas "Saint Nicholas," bishop of Asia Minor who became a patron saint for children. Now a worldwide phenomenon (Japanese santakurosu). Father Christmas is attested from 1650s.
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Definitions of Santa from WordNet
the legendary patron saint of children; an imaginary being who is thought to bring presents to children at Christmas;