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. Most silicon chips integrating these cores will package them as modified Harvard architecture chips, combining the two address buses on the other side of separated CPU caches and tightly coupled memories.
There are two subfamilies, implementing different ARM architecture versions.
Read more about ARM9: Differences From ARM7 Cores, ARM9TDMI-based Cores, ARM9E-based Cores, ARM9-based Chips, ARM9-based Products