Computronium is a material hypothesized by Norman Margolus and Tommaso Toffoli of the Massachusetts Institute of Technology to be used as "programmable matter," a substrate for computer modeling of virtually any real object.
It also refers to a theoretical arrangement of matter that is the best possible form of computing device for that amount of matter.
Computronium in popular culture 
In the Hitchhiker's Guide to the Galaxy books, by Douglas Adams, the entire Earth is portrayed as programmable matter; a computer designed to find the question to the Answer to Life, the Universe, and Everything.
In Orion's Arm Universe Project computronium is any form of matter which supports computation, including the human brain.