What defines a spring in natural terms?

A spring is defined in natural terms as a point where water flows from underground to the surface. This phenomenon occurs when groundwater, usually from aquifers, is pushed upward due to pressure or geological formations, creating a continuous flow of water at the surface. This process is essential in maintaining ecosystems and providing fresh water to various habitats.

The other options represent different water bodies or processes but do not fit the definition of a spring. Water pooling in a lake refers to standing water bodies that collect from various sources but does not involve the movement from underground. Water vapor escaping from the earth pertains to evaporation or geological outgassing and does not signify the movement of liquid water. Collecting in a reservoir involves the artificial accumulation of water, typically for specific human use, such as drinking water supply or irrigation, rather than a natural flow from underground sources.