In Keen 1-3 a metasprite is something that behaves like a sprite, but strictly speaking isn't one. They are treated specially by the game, with only 16 allowed in any one level. Metasprites are used for the falling block, opening doors, icecubes, the Mangling Machine and more.
While metasprites can be said to exist in Keen Galaxy, these are in fact normal sprites that lack an animation, animations being optional in those games.
In essence they are something invisible that produces a visible effect. They are used often when something needs to be done regularly (Icecube creation) or tiles need to be affected in an orderly manner (The destruction sequences of the Mangling Machine and Tantalus ray.) The importance of metasprites comes from the limited number allowed; if the level's quota is reached, doors will not show proper opening animations and other bug will occur related to low memory. If any of these bugs are seen then it is a sure bet metasprites are the problem.
The following are confirmed uses of metasprites: