Emptiness' or 'voidness' is an expression used in Buddhist thought primarily to mark a distinction between the way things appear to be and the way they actually are, together with attendant attitudes which are held to be spiritually beneficial.
Emperor Wu: "I have built many temples, copied innumerable Sutras and ordained many monks since becoming Emperor. Therefore, I ask you, what is my merit?"
Bodhidharma: "None whatsoever!" answered Bodhidharma.
Emperor Wu: "Why no merit?"
Bodhidharma:: "Doing things for merit has an impure motive and will only bare the puny fruit of rebirth."
Emperor Wu, a little put out: "What then is the most important principle of Buddhism?"
Bodhidharma: "Vast emptiness. Nothing sacred."
Emperor Wu, by now bewildered, and not a little indignant: "Who is this that stands before me?"
Bodhidharma: "I do not know."
This "emptiness" was neither absence nor a void. . . Like the empty mirror on which all things pass, leaving no trace, this ku contains all forms and all phenomena, being a symbol of the universal emphasis. Thus this emptiness is also fullness, containing all forms and phenomena.