Cells that do not have a well-defined nucleus and other membrane-bound organelles are called prokaryotes.
Bacteria and archaea are the two types of prokaryotes.
There are no multicellular prokaryotes.
Bacteria and archaea are the only unicellular prokaryotes to exist and there has been no evidence of the existence of multicellular prokaryotes in the past.
There are bacteria that aggregate to form colonies but they are not considered multicellular.