archaebacterium
noun
ar·chae·bac·te·ri·um ˌär-kē-ˌbak-ˈtir-ē-əm 
: any of the microorganisms comprising the archaea : archaean
Note: The archaea were originally classified with the bacteria. They have since been placed in their own domain, and use of the term archaebacterium is now typically considered outmoded.
Word History
Etymology
New Latin
First Known Use
1977, in the meaning defined above