ARM9 is an ARM architecture 32-bit RISC CPU family. With this design generation, ARM moved from a von Neumann architecture (Princeton architecture) to a Harvard architecture with separate instruction and data buses (and caches), significantly increasing its potential speed. The ARM9 processors are well suited for applications requiring a mix of DSP and microcontroller performance.
More information on ARM9 processors can be found on ARM's website and this Wikipedia article (excerpt based on them)
