In 1972, M. J. Flynn devised a taxonomy of parallel computers. Flynn looked at how many instruction streams and how many data streams there were in an application. An instruction stream is basically a program executing one instruction after another, while a data stream is a set of data values that are being operated upon. The traditional von Neumann computer has only one of both, so it is called SISD, or Single Instruction Single Data.
Here is the complete Flynn taxonomy:
One of the interesting things about Flynn's taxonomy, which was really meant to contrast SIMD, MIMD and traditional computers, is that it predicts a MISD computer. No one has built such a computer nor are they likely to, although some computer scientists consider a pipelined processor to be a MISD computer.
