What prerequisite for life as we understand it is key to habitability searches?

Liquid water existing on the surface

The search for life immediately focuses on environments where liquid water can persist on the surface, as this substance is universally considered the fundamental prerequisite for life as currently understood by science. While life might exist in subsurface liquid environments, the classic definition of habitability, especially concerning the Habitable Zone, centers on conditions that allow water to remain in its liquid state rather than being perpetually frozen solid or boiled away into vapor. This requirement guides the initial screening process for both Solar System bodies and exoplanets.