What is the common feature of domain archea and domain bacteria

Archaea and bacteria are bothprokaryotes, meaning they do not have a nucleus and lack membrane-bound organelles. They are tiny, single-cell organisms which cannot be seen by the naked human eye calledmicrobes. When we look at them through a microscope, we find that archaea and bacteria resemble each other in shape and size. They exist as rods, cones, plates, and coils. Both archaea and bacteria have flagella, thread-like structures that allow organisms to move by propelling them through their environment.

It's cell theory. easy name i would think.