Can be unique to the particular device.

Most of the time it means some form of pull up, or pull down (if driver to the pin is tristate it fgoes to that state) or a "keeper" which means that the input is  weakly held in a particular state, such that nothing is connected outside the IC but the pin is kept either high or low.

The details are unique to a device. If you look at a FPGA/CPLD/uController document, these definable states will have explicit definitions. (i.e. "5 uA pull up current is turned on when the I/O is configured in the Keep High mode)

unique to vendor-design

RTFM