BLACK ART
Technique to bring objects to life against a black background, using black-clad performers.
Precise lighting of the object (puppets etc) only, with no light on the performers, ensures the illusion is flawless.
See also Ultraviolet.

COLD READING
1) A series of techniques used by mentalists, psychics, fortune-tellers, and illusionists to determine or express details about another person, often in order to convince them that the reader knows much more about a subject than they actually do.
2) A readthrough of the play with no preparation.
Cold Reading during an audition

DOUBLE
A member of the cast or crew who appears on stage in place of the leading actor for a particular moment or sequence, either because the lead actor is doing a costume-change, or is in a different part of the venue at the time. A double is also used for some illusion sequences where the lead actor 'magically' appears to have moved an impossible distance in a short space of time. 
See also Stunt Performer. 

FLASH PAPER
Flash paper is fast-burning nitrocellulose (Cellulose Nitrate) which is used by stage magicians to produce a flash of flame safely. The paper (or cotton) made from nitrocellulose burns almost instantly leaving no ash or residue.
Flash Cotton is cotton wool / string impregnated with nitrocellulose. It should be stored wet, and allowed to dry out just before use.
Wikipedia entry

ILLUSION
The definition of Illusion is that it is a distortion of the senses, taking advantages of weaknesses in the ways the human brain organises and interprets sensory stimulation. In our world, it's a practical effect on stage which makes it appear that something impossible has taken place.  
See also BLACK ART, COLD READING, DOUBLE, GAUZE, PEPPER'S GHOST, TROMPE L'OEIL, WILD WALL

PEPPERS GHOST
Optical illusion effect used to make a ghost appear on stage next to an actor. A sheet of glass is hung across the front of the stage so that the image of an actor standing in the orchestra pit appears to float on stage. First shown at the Royal Polytechnic Institution in London by J.H.Pepper on Christmas Eve, 1862. Following many subsequent events, Charles Dickens used it during readings of The Haunted Man. Several plays were written specially to use the effect around 1863, but the long-term future of the effect was limited by the fact that the ghost couldn't speak.
Peppers Ghost is now used to great effect in smaller scale applications like the Haunted Mansion in Disney theme parks.

More about Peppers Ghost
Dark Rides at Theatrecrafts.com

PHANTASMAGORIA
A form of theatre which used a modified magic lantern to project frightening images such as skeletons, demons, and ghosts onto walls, smoke, or semi-transparent screens, frequently using rear projection. The projector was mobile, allowing the projected image to move and change size on the screen, and multiple projecting devices allowed for quick switching of different images. Invented in France in the late 18th century, it gained popularity through most of Europe (especially England) throughout the 19th century.

PLANT
1) One of the main characters in The Little Shop Of Horrors, a stage musical. The plant is named Audrey II, and there are (at least) 3 size variations on it during the course of the show, each of which needs to have various capabilities. Audrey II can be hired from scenic companies across the world. 
2) A member of the production team who is placed in the audience to take part in a key part of the show, while appearing to be a randomly selected member of the audience (also known as a STOOGE)

SLUSH POWDER
A powder used to instantly transform a liquid into a solid gel which makes the liquid seem to vanish.