In computer science and mathematics, "undecidable" refers to problems or questions that cannot be definitively resolved using an algorithm. This means there is no systematic method or procedure that can provide a correct answer for all possible inputs. A classic example of an undecidable problem is the Halting Problem, which asks whether a given program will eventually stop running or continue indefinitely. Undecidability is significant because it highlights the limitations of computation and formal systems. It shows that there are inherent boundaries to what can be solved algorithmically, influencing fields like theory of computation and mathematical logic. Understanding undecidable problems helps researchers identify which questions can be answered and which cannot.