SISD (Single Instruction Stream, Single Data Stream)

refers to the traditional von Neumann architecture where a single sequential processing unit (PU) operates on a single stream of data

SIMD (Single Instruction Stream, Multiple Data Streams)

performs the same operation on multiple data items simultaneously (used in GPUs)

MISD (Multiple Instruction Streams, Single Data Stream)

employs multiple PUs to execute different instructions on a single stream of data. This type of parallelism is not so common but can be found in pipelined architectures such as systolic arrays

MIMD (Multiple Instruction Streams, Multiple Data Streams)

uses multiple PUs to execute different instructions on different data streams

SIMT (Single Instruction Stream,
Multiple Threads)

is SIMD combined with multi-threading (i.e. each PU can take on multiple data streams)