A path from an input port to an asynchronous set or clear pin of a sequential element.
See the following fig for understanding clearly.
As you know that the functionality of set/reset pin is independent from the clock edge. Its level triggered pins and can start functioning at any time of data. So in other way we can say that this path is not in synchronous with the rest of the circuit and that's the reason we are saying such type of path an Asynchronous path.