What type of organism is defined as being made up of one cell?

A unicellular organism is defined as a life form consisting of a single cell that carries out all necessary functions for survival. This means that every aspect of biological activity, including metabolism, reproduction, and response to environmental changes, occurs within that single cell. Examples of unicellular organisms include bacteria and some protists.

In contrast, multicellular organisms are composed of many cells that often perform specialized functions. Prokaryotes refer to a category of organisms that lack a defined nucleus, which includes most unicellular organisms like bacteria, while eukaryotes are organisms whose cells contain a nucleus and other organelles. While eukaryotes can be unicellular, not all unicellular organisms fall into this category, as many prokaryotes (which are always unicellular) are not classified as eukaryotic. Thus, the specific definition of being made up of one cell directly points to unicellular organisms.