What aspect of software processes is described as defining what the system should do?

The correct answer is derived from the understanding of software engineering processes, particularly focusing on the role of specification. Specification refers to the process of documenting the requirements and functionalities that a software system must fulfill. This serves as a foundational element in software development because it outlines the expectations for the system's behavior, constraints, and the needs of the stakeholders.

By establishing a clear and detailed specification, developers and engineers have a guide to what needs to be built. This serves as a communication tool among team members and ensures that everyone has a shared understanding of the system's objectives. Without a well-defined specification, it can be challenging to assess whether the implemented software meets its intended purpose.

In contrast, implementation is about translating the defined specifications into actual code, evolution relates to the ongoing updates and maintenance of the software after its initial release, and testing involves verifying that the system meets its specifications and functions correctly. Thus, specification is distinctly focused on articulating what the system should achieve before any coding or testing takes place.