Not to be confused with the similar-sounding strudel (a famous Viennese pastry), streusel is a crumbly, crunchy dessert topping that's common in Central Europe.

A mixture of flour, butter and sugar—plus optional flavorings like cinnamon, vanilla extract, lemon zest or nuts—it's sprinkled atop coffee cakes, muffins, fruit-based crumbles and other sweets before baking. (The word is derived the German verb "to scatter.")

Streusel is also the name of a round Alsatian brioche covered in sweet shortcrust pastry and sometimes filled with cream.

Reference: What is a crumble? (Cookthink)
Recipe: Maple Plum Crumble (Cookthink)