unreachable

Unreachable is an English adjective meaning not able to be reached or attained, and it is used in various domains to describe people, places, objectives, or states that cannot be accessed under current conditions. The construction comes from un- + reachable.

In computing and formal methods, unreachable is used to describe code paths or states that cannot occur during execution or exploration. Unreachable code may arise after a return statement, after a conditional that is logically impossible, or due to design constraints. Modern compilers may issue warnings or optimize away such code. In automata theory and model checking, an unreachable state is one that cannot be reached from the initial state, indicating a possible simplification or a flaw in the model.

In networking and graph theory, reachability analyses determine whether a node or location is reachable from a given source. An element deemed unreachable may affect routing, access control, or reliability assessments.

The term also appears in culture and media as a thematic descriptor or title, connoting distance or elusiveness, though specific uses vary by work.

Common synonyms include inaccessible, unattainable, out of reach, and out of grasp. See also reachability, unreachable code, and unreachable state.