The Collaborative International Dictionary of English v.0.48:

halobacter \halobacter\, halobacterium \halobacterium\n.; pl.
   halobacteria, or halobacters.
   Any halophilic bacterium of the archaebacteria group,
   expecially of the genera Halobacterium and Halococcus,
   which live in saline environments such as the Dead Sea or
   salt flats.

   Syn: halobacteria, halobacter.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

halobacterium
    n 1: halophiles in saline environments such as the Dead Sea or
         salt flats [syn: halobacteria, halobacterium,
         halobacter]