An industrial motherboard form factor with size specifications that are approximately the same dimension of a 3.5 inch hard drive, which it is named after. It measures about 146mm x 101.6mm (5.75” x 4”). It is a very compact board size, mostly used for low power applications that are limited on space. There are some high powered 3.5 inch boards (i.e. with i7 processors) but heat becomes a concern with high powered processors, so the thermal design needs to be clever.

Word History:
You’ll note that this board isn’t actually 3.5 inches, which makes it a poor name. 3.5 Inch hard drives aren’t actually 3.5 inches wide either, they are 4 inches. This goes back to floppy disks (an early form of storage). The floppy disk stored information on a flexible magnetic disk. The disk itself was 3.5 inches wide, but the casing built around it brought the size up to 4 inches. Hence why modern 4 inch hard drives are called 3.5 inch drives. And hence why modern 3.5 inch motherboards are actually 4 inches wide.