A von Neumann machine is either of two different machines popularized by the famous mathematician John von Neumann.

General-purpose computer

A von Neumann machine is a model for a computing machine that uses a single storage structure to hold both the set of instructions on how to perform the computation and the data required or generated by the computation.

John von Neumann helped to create the model as an example of a general-purpose computing machine. By treating the instructions in the same way as the data, the machine could easily change the instructions. In other words the machine was reprogrammable.

Because the machine did not distinguish between instructions and data, it allowed a program to modify or replicate a program. These features are exploited by computer viruses when they add copies of themselves to existing program code. See von Neumann architecture.

Universal constructor

The term von Neumann machine also refers to the idea of a self-reproducing machine, which was first examined in a rigorous manner and popularized by John von Neumann who called it a "Universal Constructor." In principle, if a machine (for example an industrial robot) could be given enough capability, raw material and instructions then that robot could make an exact physical copy of itself. The copy would need to be programmed in order to do anything. If both robots were reprogrammable, then the original robot could be instructed to copy its program to the new robot. Both robots would now have the capability of building copies of themselves.

These machines could be used to explore--or conquer--the universe. The fact that we haven't seen any from other civilizations is a contributing element of the Fermi paradox. Von Neumann machines that are out of control are sometimes called "grey goo".

Since such a machine is capable of reproduction, it could arguably qualify as a life-form.

See Conway's Game of Life, John Conway.