Father Christmas vs Santa

Father Christmas and Santa both are nouns.

Word	Noun	Adjective	Verb	Adverb
Father Christmas	Yes	No	No	No
Santa	Yes	No	No	No

As nouns, Father Christmas and Santa are synonyms defined as:

Father Christmas and Santa: the legendary patron saint of children; an imaginary being who is thought to bring presents to children at Christmas

Other synonyms of Father Christmas include Kriss Kringle, Saint Nicholas, Saint Nick, Santa Claus, St. Nick.

Father Christmas (noun)	Santa (noun)
the legendary patron saint of children; an imaginary being who is thought to bring presents to children at Christmas	the legendary patron saint of children; an imaginary being who is thought to bring presents to children at Christmas