A pigmented protein embedded in the cell membrane of the bacteria Halobacterium halobium and other photosynthetic species of bacteria. The protein transports protons across the cell membrane, and its effectiveness (and thus the bacterium's effectiveness at making energy) depends on the amount of light it receives.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.