The term "subsurface ocean" refers to a hypothetical body of water that is located beneath the surface of an Earth-like exoplanet. This concept is based on the idea that some planets, similar in size and composition to our own planet, could have liquid water existing below their solid surfaces due to high temperatures or pressures. The presence of such a subsurface ocean would imply that life as we know it might be able to exist elsewhere in the universe, since liquid water is considered essential for life as we understand it.