What defines a living thing that can act or function independently?

The term that defines a living thing that can act or function independently is "organism." An organism is a complete and individual living entity that can carry out all necessary life processes, such as growth, reproduction, and response to stimuli on its own. This concept encompasses a wide variety of life forms, including single-celled organisms, like bacteria, as well as multicellular organisms, like plants and animals.

In contrast, a cell is the basic unit of life but does not function independently as a complete living entity on its own in many cases. While cells can have specific functions, they are typically part of a larger structure, like tissues or organs, within multicellular organisms.

The term "species" refers to a group of organisms that can interbreed and produce fertile offspring, but it does not describe an individual living entity. Similarly, "population" describes a group of individuals of the same species living in a specific area at the same time, but it also does not capture the concept of an independent living thing.

Thus, the definition strictly applies to "organism," as it encapsulates the idea of an independent living entity capable of functioning on its own.