Flynn's taxonomy is a classification of computer architectures, proposed by Flynn in 1972, based on the number of streams of instructions and data:

• Single instruction, single data stream (SISD) - a sequential computer, the traditional uniprocessor machines like a PC or mainframe.
• Multiple instruction/single data stream (MISD) - unusual.
• Single instruction, multiple data streams (SIMD) - e.g. an array processor.
• Multiple instruction, multiple data streams (MIMD) - multiple autonomous processors simultaneously executing different instructions on different data. This is what we call a distributed system.

There is also a common extension to this taxonomy which is:

• Single Program, multiple data streams (SPMD) - multiple autonomous processors simultaneously executing the same instruction set (but at independent points) on different data. Also referred to as 'Single Process, multiple data'.

References

• A Survey of Parallel Computer Architectures", Duncan, Ralph, IEEE Computer. February 1990, pp. 5-16.
• Flynn, M., Some Computer Organizations and Their Effectiveness, IEEE Trans. Comput., Vol. C-21, pp. 94, 1972.