Pepper's Ghost is a special effects technique for creating transparent ghostly images. This technique has been a staple of theaters and haunted houses since John Pepper popularized it in the 1800s. It works by reflecting an image of the ghost off of a sheet of plexiglass.
This special effect is especially impressive because it can be performed live and it does require any computers or special equipment. All you need to incorporate this effect into your Halloween setup is a large sheet of plexiglass.