What is the meaning of Ilmul Ghaib?

Answer

Ghaib refers to any and all things about which knowledge is not possible except if revealed by Allah Ta'ala. Thus Jannat, Jahannum, the Angels, the future and innumerable issues are unknown to man. Such unknown issues belong to the realm of ghaib, and man cannever discover such knowledge by any means other than by information conveyed/revealed by Allah Ta'ala.