When a medium is unbounded the surface which contains it is taken to be infinitely distant from the source and receiver. Radiation conditions are the Dictionary:Boundary conditions for a boundary at infinity. The result is to prevent energy from coming from infinity and to enforce the requirement that the general trend of propagation is outgoing from the source. A radiation condition is necessary to guarantee a unique solution in physical sciences problems.
One common radiation condition is called the Sommerfeld radiation condition.