Which term is defined as a living thing?

The term that accurately refers to a living thing is "organism." An organism is defined as an individual entity that exhibits the properties of life, such as growth, reproduction, response to stimuli, and metabolism. This can include a wide variety of life forms, from single-celled bacteria to complex multicellular organisms like plants and animals.

Understanding the other options helps to reinforce why "organism" is the best choice. A cell is the basic structural and functional unit of life, but it does not represent an entire living thing, as it may be a part of a larger organism. Species refers to a group of organisms that can interbreed and produce fertile offspring, which describes a category of living things rather than an individual. Population pertains to a group of individuals of the same species living in a specific area, emphasizing numbers and distribution instead of defining what constitutes a living thing at the individual level. Therefore, "organism" is the term that best encapsulates the concept of a living entity.