In futures studies, computronium refers to a hypothetical material engineered to maximize its use as a computing substrate. While futurists usually use it to refer to hypothetical materials engineered on the molecular, atomic, or subatomic level by some advanced form of nanotechnology, the term can also be applied both to contemporary computing materials, and to constructs of theoretical physics that are unlikely to ever be practical to build.
Many futurists speculate about futures where demand for computing power grows to the point where very large amounts of computronium are desired. Examples of applications include Jupiter brains, planet-sized constructs made of computronium, and matrioshka brains, concentric Dyson spheres designed to extract all possible energy from the host star for use towards computation.
Computronium is used quite a lot in science-fiction. Just in Alastair Reynolds’ Revelation Space books, there are multiple applications of this. I remember that in the Iain M. Banks book Matter, the book takes place on a shellword, a sort of Dyson sphere with multiple layers.