playoff vs World Series

playoff and World Series both are nouns.

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

As nouns, World Series is a hyponym of playoff; that is, World Series is a word with a more specific, narrower meaning than playoff:

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

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