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.
1977, in the meaning defined above
“Archaebacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/archaebacterium. Accessed 6 Dec. 2022.
Britannica.com: Encyclopedia article about archaebacterium
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