What term describes a common boundary where different parts of a system interact?

The term that describes a common boundary where different parts of a system interact is "interface." In geological contexts, an interface refers to the transitional zone where distinct materials or phenomena meet and affect each other. This could be, for example, the boundary between soil and bedrock, where soil properties might influence water movement or nutrient availability. Interfaces are critical in various systems, including ecological, geological, and hydrological, contributing to the complexity of interactions that occur at these boundaries.

Regolith refers to the layer of loose, heterogeneous material covering solid rock, while residual soil is the soil that remains in the same location where it was formed, without being transported. "Soil" itself is a broader category that encompasses the upper layer of earth in which plants grow, composed of organic matter, clay, and minerals. None of these terms adequately capture the concept of a boundary where interactions take place, making "interface" the most appropriate choice.