I've denuded many a coffee cake in my day: that streusel topping, all crumbly with sugar-and-butter pebbles just calls to me. I don't want the cake; I never want the cake. I just want the streusel.
According to the Oxford English Dictionary, the term streusel comes from the German verb streuen, which means "to sprinkle."
In it's simplest form, streusel is flour, sugar and butter, though many bakers add oats, or a little cinnamon to the topping, as Alice of Savory Sweet Life does with her sour cream blueberry coffee cake. Others, like Mary of One Perfect Bite, add nuts to their streusel for desserts like this incredible-looking apple custard pie.
My preferred streusel topping recipe has all three. Make an extra-large batch, leave it in the freezer, and use it whenever you've got a streusel craving. Take it out of the freezer, let warm up for 15 minutes (or until it's warm enough to crumble in your fingertips), and start topping.