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.
References
This article (or an earlier version of it) contains material from FOLDOC, used with permission.