Which word describes a continuous extent where parts cannot be easily distinguished?

The term "continuum" refers to a continuous sequence or range in which adjacent elements are not distinctly separate, highlighting the idea of gradual change without clear boundaries. This concept is relevant in various fields, including physics, mathematics, and philosophy, where it is used to describe phenomena that flow seamlessly into one another rather than existing as discrete, separate units. In the context of the question, "continuum" aptly captures the notion of an extent where parts cannot be easily distinguished due to their interrelated nature.

The other words do not convey the same meaning. "Onerous" describes something burdensome or difficult, which does not relate to the idea of continuous extent. "Didactic" refers to educational or instructive content, while "ephemeral" signifies something transient or short-lived, neither of which aligns with the concept of an indistinguishable continuum. Thus, "continuum" is the clear choice for this definition.