1.  Instead of physical substance the astral world has its own unique substance that some writers refer to as “emotional” because it is the foundation for our emotional consciousness just as the physical substance is the foundation for our physical consciousness, and the mental world substance is the foundation of our mental consciousness.  

A range of energy/matter substance of lessening density as the range of vibrations increases.

2.  A generic term used to describe the malleable or "plastic" substance that comprises the astral plane.