Define "context diagram".

A context diagram serves as a high-level visual representation that delineates the boundaries of a system and illustrates how the system interacts with external entities, such as users, other systems, or organizational units. This diagram is essential in understanding the overall environment in which the system operates, highlighting the inputs and outputs without delving into specific internal processes.

The primary purpose of a context diagram is to provide clarity on what is included within the system (the system's boundaries) and what lies outside of it, making it easier to communicate the system's scope to stakeholders. It effectively encapsulates the relationship between the system and its external factors, enabling stakeholders to quickly grasp how the system fits into the larger ecosystem.

In contrast, other choices do not accurately define a context diagram. For example, a summary of system functionalities focuses on the specific features and operations within the system rather than its boundaries and external interactions. A timeline of project milestones relates to project management and scheduling but does not convey information about system interactions. A document summarizing user requirements captures detailed needs and expectations from users rather than presenting a macro view of system boundaries and external relationships. Thus, the correct answer emphasizes the diagram's role in defining system scope and external interactions.