> I have never heard of "tristate" being used in this context... what does it mean?
It means that a given device is completely disconnecting itself from the bus, leaving the bus to "float" to random garbage values and/or be driven by some other device.
This is the origin of the "open bus read" commonly encountered in 8-bit computers; if you try to read an address where no device is mapped, *everything* goes tristate and the CPU reads back a semi-random value.