Plasteel is a type of advanced synthetic material that has the consistency of plastic but the tensile strength of a steel alloy that was developed during the Dark Age of Technology but long used by the Imperium of Mankind in the construction of many types of personal infantry armour.
Heavy-duty plasteel was combined with the heat-resistant ceramic material called ceramite to craft the plating of Terminator Armour when the Adeptus Mechanicus still maintained the technical knowledge required to manufacture it.
The word plasteel is actually a portmanteau of the words plastic and steel and was most notably used by author Frank Herbert in the Dune series of novels, though the concept may actually have been first coined by science fiction writer Harlan Ellison in 1956.
- Warhammer 40,000 Compendium, pg. 11