An abyss in general is something that cannot be fathomed – that is, a phenomenon that defeats any effort to explain, determine, define, rationalize it, or make it intelligible using conceptual resources. Heidegger also uses “abyss” as a term of art to refer to a specific and important type of abyss in the more general sense. “Abyss” in Heidegger’s specific sense is something that grounds the being of a thing precisely by refusing to determine it.