A Void Ship was a spacecraft designed by the Cult of Skaro to explore the Void that existed between dimensions. It appeared as a featureless floating sphere until it was activated, at which point it opened up to reveal the occupants.
When travelling through the Void, the ship was separate from time and space and, until activated, had no detectable mass, heat or radiation, though it was visible. Due to these properties people who looked upon it found it deeply unsettling. An invisible barrier prevented anything from touching the ship, at least while it wasn't active.
It was thought to be impossible by the Tenth Doctor, but it did exist. (TV: Army of Ghosts) The only Void Ship known to the Doctor was used by the Cult of Skaro to escape the Time War with the Genesis Ark. (TV: Doomsday)