A 23rd-century theory of transport by folding space between dimensions, abandoned as unworkable and harmful, which Wesley Crusher recognised as the principle behind the Ansata terrorists' untraceable inverter in The High Ground.
The Elway Theorem describes a method of moving matter by shifting it through a fold between dimensions, an approach quite unlike the ordinary transporter. It was explored in the 23rd century and then given up, since every attempt at such inter-dimensional transport proved damaging to living tissue.
The theory resurfaces in The Next Generation episode The High Ground in 2366. On Rutia IV, Ansata separatists led by Kyril Finn strike and vanish without leaving any trace that sensors can follow, even beaming through the Enterprise-D's shields to plant a bomb and to abduct Picard and Beverly Crusher. Wesley Crusher, working with Data and La Forge, realises that their device, the inverter, is an application of the forgotten theorem.
The same flaw remains: repeated use is slowly killing the Ansata, warping their cells. Once Wesley learns to trace the shifts, the base is found.