In terms of cell structure, what does unicellular mean?

Unicellular refers to organisms that consist of only one cell. This characteristic defines unicellular organisms, distinguishing them from multicellular organisms, which are made up of many cells that can work together in various structures and functions. In unicellular organisms, all necessary life functions—such as metabolism, reproduction, and response to the environment—are carried out within that single cell.

This simplicity allows unicellular organisms, like bacteria and some protists, to thrive in diverse environments despite their small size. Their entire biological processes depend on the properties and capabilities of that single cell, making the concept of unicellularity essential to understanding cellular biology and the diversity of life forms.