World Series vs playoff

World Series and playoff both are nouns.

Word	Noun	Adjective	Verb	Adverb
World Series	Yes	No	No	No
playoff	Yes	No	No	No

As nouns, playoff is a hypernym of World Series; that is, playoff is a word with a broader meaning than World Series:

World Series: series that constitutes the playoff for the baseball championship
playoff: any final competition to determine a championship

Other hypernyms of World Series include series.

World Series (noun)	playoff (noun)
series that constitutes the playoff for the baseball championship	any final competition to determine a championship