What is the term for the boundary that separates a system from its environment?

The term that describes the boundary separating a system from its environment is commonly referred to as a boundary. In the context of systems thinking and design, a boundary defines what is considered part of the system and what lies outside it. This differentiation is crucial because it helps in understanding how a system interacts with external factors, influences, and stakeholders, effectively delineating the scope of the system's functionality and responsibilities.

In software design and development, establishing clear boundaries is essential for defining the limits within which the system operates and ensuring that external interactions are managed appropriately. This concept allows developers to focus on the interactions that occur at the boundary, facilitating better system analysis and design.

While the terms "interface," "border," and "threshold" may relate to concepts in systems and software development, they do not encapsulate the primary idea of separating a system from its environment in the same way. An interface typically refers to a specific point of interaction, while "border" and "threshold" can imply different meanings that don’t specifically convey the notion of a system-environment division. Therefore, the term "boundary" is the most accurate choice for this definition.