A classification of computer architectures based on the number of streams of instructions and data:

single instruction/single data stream (SISD) - a sequential computer.

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.

[Flynn, M. J., "Some Computer Organizations and Their Effectiveness", IEEE Transactions on Computing C-21, No. 9, Sep 1972, pp 948-960].

["A Survey of Parallel Computer Architectures", Duncan, Ralph, IEEE Computer, Feb 1990, pp 5-16].