What characteristics define bacteria?

There are three notable common traits of bacteria, 1) lack of membrane-bound organelles, 2) unicellular and 3) small (usually microscopic) size. Not all prokaryotes are bacteria, some are archaea, which although they share common physicals features to bacteria, are ancestrally different from bacteria.

Bacteria are like eukaryotic cells in that they have cytoplasm, ribosomes, and a plasma membrane. Features that distinguish a bacterial cell from a eukaryotic cell include the circular DNA of the nucleoid, the lack of membrane-bound organelles, the cell wall of peptidoglycan, and flagella.