What term refers to the essential or central part of something, often indicating strength or vigor?

The term that best refers to the essential or central part of something, indicating strength or vigor, is "pith." Pith traditionally describes the central, most vital part of a plant or fruit, but it has broader applications in language, often symbolizing the essence or strength of an idea or argument. When we speak of the "pith" of a matter, we are referring to its most important, substantial, and vigorous components. Hence, using "pith" underscores the idea of robustness and centrality within the context of both physical and abstract notions.

While "core," "essence," and "center" are also related to the idea of something being central or fundamental, "pith" specifically evokes the notion of vigor and substance in a way that highlights its intrinsic strength, making it particularly suitable in this context.