Material whose radial tension exceeds its total mass-energy density, the condition identified in the Morris-Thorne Wormhole paper as necessary to hold a traversable wormhole throat open. Michael Morris and Kip Thorne coined the term in 1988 for exactly this property. No such material is known; the Casimir effect is the usual cited instance of the underlying energy-condition violation, at magnitudes vastly below what a macroscopic throat would require, and later work restated the requirement as a violation of the averaged null energy condition. It is the shared bottleneck of every geometric propulsion proposal, including the Alcubierre Warp Drive and the Metric Engineering literature.
Connections
Morris-Thorne Wormhole · Alcubierre Warp Drive · Metric Engineering · Engineering the Vacuum · Zero-Point Energy · Negative Mass Propulsion