A miniport is a type of hardware-driver, part of the Windows Driver Model. These are USB, Audio, SCSI and network card adapters. They should usually be source and binary compatible between Windows 98 and Windows 2000 and are hardware specific but control access to the hardware through a specific bus class driver.
"Introduction to the Windows Driver Foundation". Microsoft Developer Network. 2006-10-13. Retrieved 2010-12-06. "The device class-specific driver models are typically structured as a port driver written by Microsoft paired with a miniport driver written by an independent hardware vendor. The port driver does much of the work required for the device class, and the miniport driver supports device-specific characteristics."