Saint Nicholas vs St. Nick

Saint Nicholas and St. Nick both are nouns.

Word	Noun	Adjective	Verb	Adverb
Saint Nicholas	Yes	No	No	No
St. Nick	Yes	No	No	No

As nouns, Saint Nicholas and St. Nick are synonyms defined as:

Saint Nicholas and St. Nick: the legendary patron saint of children; an imaginary being who is thought to bring presents to children at Christmas

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

Saint Nicholas (noun)	St. Nick (noun)
a bishop in Asia Minor who is associated with Santa Claus (4th century)	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