What type of organism strictly consists of a single cell?

A unicellular organism is defined as an organism that consists of only one cell. This single cell carries out all the functions necessary for the organism's survival, including metabolism, reproduction, and response to the environment. Examples of unicellular organisms include bacteria, yeast, and some protozoa.

While prokaryotic and eukaryotic organisms can be unicellular, the term specifically refers to the characteristic of having a single cell. Prokaryotic organisms, like bacteria, lack a true nucleus and membrane-bound organelles, whereas eukaryotic organisms can be either unicellular or multicellular and possess a well-defined nucleus and other organelles. Therefore, while prokaryotic and eukaryotic organisms encompass a wide range of single-celled entities, the term "unicellular" directly describes the essential quality being asked in the question.