Inflatable starship decoy
From Memory Alpha, the free Star Trek reference.
An inflatable starship decoy is an extremely large closed piece of material that, when filled with a gas, has the visual appearance of a starship. The USS Enterprise computer once used such a device as to play a practical joke on some Romulans. It was 20 times the size of a Romulan battle cruiser. (TAS: "The Practical Joker")
