IWM is the official designation for what is still debated being called either the "Incredible Woz Machine" or the "Integrated Woz Machine".
The IWM is a disk controller chip that reared it's head on the Disk II floppy controller in 1983. Previously in the first Apple II drive controller, Woz had worked down the drive controller to a state of tenseness where it only required 8 chips where where there was previously required to be over 20. The '83 model of the the Disk II took this a step further by using a chip designed by Woz himself to handle most of the work which them got the total required chip count from 8 down to 3.
The IWM was such a versatile chip that is was used in models as late as the Mac II before the SuperDisk drives because standard.