Here's why:

* Hierarchy of Biological Organization: The study of life is organized in a hierarchical manner, starting with the smallest units and building up to the largest. Here's the general order:

* Atom: The basic unit of matter.

* Molecule: Two or more atoms bonded together.

* Organelle: A specialized structure within a cell (like a mitochondrion).

* Cell: The basic unit of life.

* Tissue: A group of similar cells working together.

* Organ: A structure made up of different tissues that perform a specific function (like the heart).

* Organ System: A group of organs that work together (like the circulatory system).

* Organism: A complete living being made up of organ systems.

* Population: A group of organisms of the same species living in the same area.

* Community: All the different populations of organisms living in a particular area.

* Ecosystem: A community of organisms and their physical environment.

* Biosphere: The part of Earth that supports life.

Therefore, an organism is a single, complete, and independent living entity.