topping vs streusel

topping and streusel both are nouns.

Word	Noun	Adjective	Verb	Adverb
topping	Yes	Yes	No	No
streusel	Yes	No	No	No

As nouns, streusel is a hyponym of topping; that is, streusel is a word with a more specific, narrower meaning than topping:

Other hyponyms of topping include frosting, ice, icing, glaze, meringue, whipped cream.

topping (noun)	streusel (noun)
a flavorful addition on top of a dish	a crumbly topping for a pastry
	pastry with a topping of streusel

topping (adjective)	streusel (adjective)
excellent; best possible