Which word describes the essence of something?

The term that best describes the essence of something is "pith." This word refers to the core, substance, or most important part of an object or concept. In various contexts, it conveys the idea of what is fundamental and vital, stripping away any extraneous or superficial elements. The essence encapsulates the intrinsic qualities that define the thing itself, making "pith" the most appropriate choice in this case.

Contrastingly, the other options relate to outer appearances or superficial aspects. "Surface," "facade," and "outer layer" indicate the outward or exterior elements, which do not capture the true nature or depth of an entity. These terms emphasize what is observable rather than what is foundational, which is why they do not effectively convey the idea of essence as "pith" does.