Unimplementable

Unimplementable is an adjective used to describe a plan, requirement, or design that cannot be realized in practice under a given set of constraints. These constraints can be technical, financial, temporal, legal, ethical, or organizational. The label is typically applied during requirements analysis, systems design, or project reviews when a proposed solution cannot be delivered as stated.

In software and systems engineering, unimplementability often stems from incompatible requirements, insufficient interoperability, or dependence on unavailable technologies. Examples include a feature that contradicts safety standards, a performance target that cannot be achieved within budget, or an architecture that cannot accommodate future extensions without a complete rewrite. The term signals a decision point: to revise, defer, or abandon the requirement.

Unimplementable is related but distinct from terms such as unfeasible, impracticable, and impossible. Unimplementable emphasizes feasibility of execution within the current constraints; unfeasible may refer to practicality; impossible refers to logical or physical impossibility. In theoretical contexts, a design might be labeled unimplementable if no consistent set of requirements, resources, and technologies can realize it.

Assessment approaches include feasibility studies, constraint analysis, requirement tracing, and risk assessment. Mitigation may involve negotiating scope, redefining goals, selecting alternative architectures, prototyping, or staged implementation. In some cases, an unimplementable requirement is retained as a future target but with explicit dependencies and a plan to remove contradictions or acquire new capabilities.

The notion also appears outside engineering, indicating plans or policies that cannot be enacted under prevailing conditions. It underscores the importance of clear, testable requirements and ongoing re-evaluation of project goals as technology and context evolve.