A void is one of many things.
First an foremost, and void is a type of invalidation that occurs whenever a person does something that conflicts with some sort of contractual requirement.
A void is also an expanse of empty space.
Some might say that a void is a "pit without walls or a bottom". Others say that (as far as pits go) a void is actually the empty space between the walls of the pit. One might also say that a pit that doesn't contain anything contains a void, or a man standing on the floor of a giant empty pit might call the empty field a void.
A void is sometimes seen as a special state of the bottomless pit.