A function is considered unique if, for each input x, there is exactly one corresponding output y. This definition is crucial in mathematics, particularly in the context of functions and differential equations. The uniqueness property indicates that a specific problem has only one solution that meets given conditions. For instance, the differential equation y" = -y with boundary conditions y(0) = 0 and y(1) = 1 has a unique solution, which is y(x) = sin(x). Understanding uniqueness is essential for solving mathematical problems effectively.