: The defining feature is the topping, traditionally made by rubbing cold butter into flour and sugar until it resembles coarse breadcrumbs.

: Most crumbles begin with seasonal fruits like apples, rhubarb, or berries. The fruit is often lightly tossed with sugar and spices like cinnamon or nutmeg before being placed in a baking dish.

In a broader sense, to describes the process of breaking into small fragments or decaying gradually. The BEST apple crumble recipe .. from Good Food!