Cells that do not have a distinct nucleus are called prokaryotic cells.

Here's why:

* Prokaryotic cells are simple, single-celled organisms like bacteria and archaea. They lack a membrane-bound nucleus and other membrane-bound organelles.

* Eukaryotic cells, on the other hand, are more complex cells found in plants, animals, fungi, and protists. They have a true nucleus that encloses their genetic material (DNA).

So, the defining characteristic of prokaryotic cells is the absence of a distinct nucleus.