Dreamstuff

Dreamstuff is literally the stuff of which dreams are made. Somehow, the Martians had learned how to create it in the Dream-World, and use it to create various objects in their dreams.

This became very important in Martian Dreams, where the evil Raxachk had himself hidden behind an impenetrable crystal barrier in the Dream-World. Raxachk gloated that even if the Avatar should manage to gain access to his hoarded Dreamstuff, using it to create a weapon powerful enough to bypass the barrier would be impossible for the hero, as Raxachk's psychometric powers had already revealed everything that could be known about everything possessed by the humans on Mars.

Raxachk was proven wrong when the Avatar managed to get the Dreamstuff and used it (after some imagination raising) to create a weapon from the hero's present, an M60 Machine Gun. Against this weapon, Raxachk had no chance.