To describe someone who just passed out, the choice between subconscious and unconscious is easy—the hapless fainter is unconscious. But in other situations that call for one of these words to be used, the answer isn’t always so clear
The popularity of these terms traces back to Sigmund Freud, who, according to Dr. Michael Craig Miller, editor at Harvard Health Publishing, used them as synonyms in his early work on levels of consciousness. These levels are often visualized as an iceberg, with the conscious (thoughts, feelings, motivations, and other things you’re totally aware of) depicted above the water, and the unconscious—repressed memories, desires, and other traumatic and/or taboo things to which you’re oblivious—unseen below the surface.
Freud later abandoned the word subconscious in favor of unconscious, and eventually sort of revamped his whole consciousness theory by introducing the concepts of id, ego, and superego. But he also established an intermediate level located between the conscious and unconscious minds: the preconscious. Freud believed that although you’re not actively cognizant of whatever’s stored in your preconscious, it is available for recall upon reflection.
What some people today label as the subconscious is actually Freud’s preconscious; others even cite subconscious and preconscious as interchangeable. If you’re operating off Freud’s original intent, however, that’s not accurate—and according to Miller, experts in psychology, neurology, psychiatry, and other relevant fields typically forgo the term subconscious in scientific literature altogether.
That said, Freud didn’t invent the words subconscious or unconscious, and they’re not confined to scientific circles. Colloquially, calling an emotion “unconscious” might imply to your listeners that it’s buried a bit deeper than a subconscious one. But as Grammarist explains, you can pretty much use either term to describe a lack of awareness.