BI: Backside Illuminated
A CCD chip (on ACIS) in which X-rays are incident on the silicon substrate, which has been thinned to allow X-rays through. This gives good low energy response, but leads to lower energy resolution (ignoring the effects of CTI on the FI chips). In contrast, FI (frontside illuminated) chips have the X-rays incident through the gate structure on the front of the chip.
See the ACIS entry for more details on the CCD chips.