What is the primary characteristic of protozoa?

Protozoa are primarily defined by being single-celled organisms. This characteristic distinguishes them from other categories of life forms, such as multicellular organisms like plants and animals. The single-celled nature of protozoa allows them to perform all necessary life functions—such as feeding, reproduction, and movement—within one cell, making them highly adaptable and efficient. Unlike multicellular organisms that have specialized cells and tissues to carry out various functions, protozoa maintain a simpler structure, which enables them to thrive in diverse environments, from freshwater to soil and even within other organisms as parasites. Their cellular organization is fundamentally different from organisms that are complex in body structure, which typically consist of multiple cells arranged into tissues and organs. As such, the focus on their unicellular composition is critical in understanding their biology and ecological roles.