Single Instruction, Multiple Data
SIMD is a type of instruction capable of processing several data chunks at once. For example, using a 32-bit register to load and execute four eight-bit data at the same time. In this example, this task would be four times faster than manipulating the four eight-bit data individually.
There are several implementations for SIMD technology, like MMX, SSE and 3Dnow.
To use this technology, the processor must have a SIMD instruction set and the program must be compiled (i.e. written) using it in order to have a performance gain.