What is the difference between crumble and streusel?

“While I imagine some purists will argue with me, I feel comfortable saying that there is no clear, distinct difference, at least in present-day America. Streusel originated in Germany and was made by cutting butter into a mixture of flour and (white) sugar. Over the years, many people (including myself) have made subtle variations to improve upon the base recipe to their taste preferences, and in the US, streusel is often referred to as “crumb topping”. Whatever you choose to call it, you won’t notice much of a difference in taste or texture either way.”