"Computronium" can be defined as any form of matter which supports computation, especially artificial substances suitable for high efficiency computation. Biological neural matter is also a form of computronium; baseline humans carry about 1.5 kilos of computronium in their heads as brain tissue.
It is a catchall term for a wide range of substances, the most advanced of which consist of very densely packed computational elements and used by higher intelligences for its speed and efficiency. Using it reduces signal times because of the short distance between each element and the fact that the huge number of processing units per unit volume (from a baseline's macroscale point of view, it would appear to be a continuous substance, much like human brain matter) means only a small volume of it is necessary to do a certain amount of work.
Although similar in basis, computronium varies widely according to design, function, characteristics, manufacturer, and from job to job. e.g. something that has large data access requirements (object recognition) have relatively little in common with computronium designed for something with massive computational needs (e.g. cryptography) etc. Every task requires different resources to complete, and the computronium would be optimized for performing a given type of work. Higher-order systems and AIs would no doubt have many subsystems, each containing different types of computer matter. Different empires, polities, and archai also may have a preferred type.
Biological computronium is often used by the Zoeific Biopolity; the most familiar form of biological computronium is the animal brain, particularly the naturally evolved human brain. The Biopolity and a number of other biotech-based polities use large or very large genetically engineered brain tissue processors, taking advantage of the naturally good processing power afforded by brain tissue. DNA and other complex molecules can be used in various forms of organic computronium systems.
Many other polities use the efficient and versatile Ultimate Chip system, which allows processing many orders of magnitude smaller than biotech processors.
Like all computer equipment, there are many different grades and implementations, ranging over all levels of technology, from regular nanoscale down to the smallest limits of matter manipulation. Smaller scales increase heat dissipation problems, of course, but also means the means of dealing with them is more advanced -- regular hi-tech computronium can be cooled by passive radiative means, for instance, whereas the densest forms of godtech computronium requires more advanced refrigeration such as neutrino cooling.
Computation - Text by M. Alan Kazlev Basically, what a computer does; which is mapping one set of numbers to another. The actual process of computing can be defined in terms of a very small number of very simple operations, such as addition, multiplication, recursion, and so on. Computing devices can also make statements about other computing devices.
Computer - Text by M. Alan Kazlev A computation device, which may be sentient or non-sentient. AIs can be either sentient computers, or complex systems residing in the RAM of computers. Without computers, galactic civilization would be impossible.
Computer Science - Text by Adapted by M. Alan Kazlev from the original write-up by Robert J. Hall The theoretical and practical application of computational devices, including hardware, firmware and software architecture, programming skills, simulation techniques, customized algorithms and dedicated aioids, virtualics, networking, parallel programming, intelligent subroutines, information storage, comparative machine-level low level, and intermediate level operating systems, and subturing artificial intelligences.
DNA Computing - Text by M. Alan Kazlev A form of computing dating back to the Information Age, in which DNA molecules are used to solve complex mathematical problems or generate virtual worlds. DNA computers allow trillions of computations to be performed simultaneously. However, they are slower than standard nanocomputers.
Molecular Computer - Text by M. Alan Kazlev Any computer based on logic gates that is constructed on principles of molecular mechanics (as opposed to principles of electronics) by appropriate arrangements of molecules. Since the size of each logic gate is only one or a few molecules, the resultant computer can be microscopic in size. As with any nanotech, limitations on molecular computers arise from the physics of atoms and chemical bonds. Molecular computers are massively parallel through having parallel computations performed by trillions of molecules simultaneously. The early molecular computers (Middle Information Age) were constructed from the DNA molecule; later on, more adaptable artificial organic and artificial organic molecules were used instead.
Optical Computer - Text by M. Alan Kazlev A computer that processes information using light instead of electrons. Each stream of photons represents an independent sequence of data, thereby providing extremely massive parallel computation. Nanooptical computers can use a single photon as a bit.
Ubiquitous Computing - Text by Anders Sandberg in his Transhuman Terminology Also known as "embodied virtuality". Information and early Interplanetary Age Old Earth. Computers that are an integral, invisible part of people's lives. In some ways the opposite of virtual reality, in which the user is absorbed into the computational world. With ubiquitous computing, computers take into account the human world rather than requiring humans to enter into the computer's methods of working. Ubiquitous computing evolved into the micro-, meso- and micro/meso/nanon based interactivity of the middle Interplanetary Age and later.
Universal Computer - Text by M. Alan Kazlev An idealized hypothetical computer that is capable of universal computation, which means that it is capable of computing anything that can in principle be computed. Given a description of any other computer or program and some data, it can perfectly emulate this second computer or program.