What is the term for a small, living plant or animal that is not visible to the naked eye?

The term for a small, living plant or animal that is not visible to the naked eye is "microorganism." This category encompasses a broad range of tiny life forms, including bacteria, viruses, and protozoa, that are typically measured in micrometers. Microorganisms play significant roles in various ecosystems, including human health, agriculture, and biotechnology.

While bacteria are a specific type of microorganism, the term "microorganism" itself includes various forms of life beyond bacteria, such as viruses and protozoa. This term is inclusive of all microscopic entities, making it the most accurate choice for describing small living organisms that cannot be seen without the aid of a microscope. It reflects a broader classification that encompasses all the options listed in the question.