What is the term used to describe the non-buttered side of the toast?

The correct term used to describe the non-buttered side of the toast is often referred to as the "flat" side. When preparing toast, the side that is buttered typically has a smooth and shiny appearance due to the spread, while the non-buttered side remains a bit rougher and drier, thus it can be described as "flat."

In this context, describing the non-buttered side as "flat" conveys its unadorned characteristic compared to the embellished buttered side. Other terms such as "concave," "convex," or "curved" might describe shapes or surfaces in different contexts, but they don't accurately characterize the texture or appearance of the non-buttered side of the toast in this scenario.