That question always reminds me of Heidegger and Sartre getting into a fight over what Heidegger meant by “exist.”
It’s a tough word to define without being circular. I’d say that to exist in the broadest sense is to be actually real in some form or other. For example, the plant on my desk exists; it has physical properties, it takes up space, it exists in time, etc. On the other hand, unicorns (sadly) (presumably) do not actually exist, because you can’t find a thing out in the world that satisfies the definition of a unicorn. I can imagine a unicorn, but that only means the idea of a unicorn exists. The unicorn itself, the “formal reality” of the idea, isn’t real.
And then, of course, there’s Heidegger, who says that Dasein’s essence is to ek-sist, or to stand outside itself. But that’s another story.
