What term is used for a direct comparison between two unrelated subjects without using "like" or "as"?

The term that refers to a direct comparison between two unrelated subjects without using "like" or "as" is indeed metaphor. A metaphor asserts that one thing is another, thereby highlighting similarities between the two subjects in an implicit manner. For example, saying "Time is a thief" conveys that time stealthily takes away moments in a manner analogous to how a thief would steal. This comparison invites readers to explore the deeper association between the concepts without relying on comparative language.

In contrast, a simile explicitly uses "like" or "as" to make a comparison, which means it falls outside the definition of a metaphor. Metonymy involves substituting the name of one thing with the name of something else closely associated with it, rather than drawing a direct comparison. An analogy is a broader term that refers to a comparison that explains a concept through its similarities to another, often used to clarify or illustrate ideas, but it doesn't necessarily fit the specific framework of a direct comparison without the comparative language that a metaphor does.