An organism such as a bacterium that consists of a single cell with no nucleus is called a prokaryote.

Here's why:

* Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and other membrane-bound organelles. They have a simpler structure than eukaryotic cells.

* Eukaryotes, on the other hand, are organisms whose cells have a nucleus and other membrane-bound organelles.

Bacterium is a classic example of a prokaryotic organism.