Which term is used to describe a situation that has reached a dead end?

The term that best describes a situation that has reached a dead end is "impasse." An impasse occurs when progress is halted due to conflicting parties or circumstances that prevent any further movement toward resolution. This term is often used in negotiations or discussions where neither side is willing or able to make concessions, resulting in a stalemate.

In contrast, a truce refers to a temporary halt in fighting or a cessation of hostilities, which does not imply the same sense of being stuck as an impasse does. A solution signifies an answer or resolution to a problem, indicating successful progress rather than a standstill. A breakthrough suggests a significant and sudden development that overcomes obstacles, highlighting advancement rather than stagnation. Therefore, "impasse" is the most fitting choice for describing a situation where no progress can be made.