ARM - Acorn Risc Machine

ARM is a RISC processor with the following features
Load-store architecture
16 registers, each 32-bits wide
Three-operand instructions, 32-bits wide
At each clock cycle a new instruction is issued
Low energy consumption (W/MIPS)

It is designed by ARM Ltd. and licensed to manufacturers
ARM7: nommu, very widespread as a microcontroller
StrongARM: DEC, then Intel (now dead)
Xscale: Intel, then Marvell (80200, PXA255, PXA270, IXP425, ...)
EP93xx: Cirrus Logic (es: 9302, 9315)
iMX: Freescale (iMX1, iMX21, iMX27, iMX31, iMX51)
AT91: Atmel
... many more ...

http://en.wikipedia.org/wiki/ARM_architecture