Final answer:
Organisms in the domain Archaea are unicellular and lack membrane-enclosed organelles and a cell nucleus, with distinctive lipid compositions in their cell membranes.
Step-by-step explanation:
A characteristic of something in the domain Archaea is that it is unicellular. Like organisms in the domain Bacteria, organisms of the domain Archaea are all unicellular organisms and lack membrane-enclosed organelles and a cell nucleus, which are typical features of eukaryotic cells. Archaea are prokaryotes and have a unique lipid composition in their cell membranes, with phytanyl units linked to glycerol. Some archaeal membranes are lipid monolayers instead of the typical bilayer found in other organisms' cells.