What do we call a comparison between two things?

A comparison between two things is known as an analogy. An analogy expresses the relationship between two different concepts or items, often highlighting similarities in structure or function to help clarify an idea. For example, saying "a detective is to a mystery as a doctor is to a disease" draws parallels between the roles and functions of a detective and a doctor in their respective contexts, making it easier to understand the significance of each role through the comparison.

The other options do not fit the definition of a comparison. "Effect" refers to the result or outcome of an action, while "transient words" are not a recognized term in this context. "Narrative" pertains to storytelling and the recounting of events rather than making comparisons. Thus, the correct term for a comparison between two entities is indeed analogy.