* Eukarya: This domain encompasses all organisms whose cells have a true nucleus and other membrane-bound organelles. Multicellular organisms, by definition, are made up of many cells, and these cells are eukaryotic.

* Bacteria: This domain consists of prokaryotes, which are single-celled organisms lacking a nucleus and membrane-bound organelles.

* Archaea: This domain also consists of prokaryotes, but they are distinct from bacteria in their biochemistry and genetics.

Therefore, the presence of eukaryotic cells is the defining characteristic that places multicellular organisms within the domain Eukarya.