Describing a living entity accurately can be challenging. In biology, an organism is defined as a life form that can respond to stimuli, grow, reproduce, and maintain internal balance.

Classification systems bring order to the millions of diverse life forms on Earth. The roots of modern taxonomy trace back to ancient Greek scholars and Aristotle, who first grouped plants and animals based on observable traits.

Definition and Core Characteristics

An organism is a single, living individual. It can be a simple, single‑cell organism such as a bacterium, or a complex, multicellular entity whose parts cannot survive independently. According to Merriam‑Webster’s online dictionary, an organism is "an individual living being that carries out life functions via interdependent organs."

In 1753, Swedish naturalist Carolus Linnaeus formalized the system of binomial nomenclature and hierarchical classification that still underpins biological science. The Linnaean framework enables scientists worldwide to communicate findings without exhaustive descriptions, while new terms continue to be coined for newly discovered species.

Domains of Life

Life is divided into three primary domains, each defined by genetic and cellular distinctions:

• Eukarya – organisms with a membrane‑bound nucleus. This domain includes protists, fungi, plants, and animals, all of which possess organelles and a cytoskeleton.
• Archaea – prokaryotic organisms lacking a nucleus. Often extremophiles, archaea thrive in harsh environments such as hot springs (thermophiles) or high‑salt habitats. Methanogens, for example, produce methane in anaerobic settings like sewage treatment plants.
• Bacteria – prokaryotic organisms without a nucleus. Carl Woese’s 1970s work revealed that bacteria and archaea are genetically distinct, each with unique genetic codes.

Kingdoms and Phyla

Domains are further subdivided into kingdoms. The former kingdom Monera has been split into two: Archaebacteria and Eubacteria. The six widely accepted kingdoms are:

• Archaebacteria
• Eubacteria
• Protists
• Fungi
• Plants
• Animals

Kingdoms are then partitioned into phyla. The animal kingdom alone contains nearly thirty phyla, with Arthropoda—the most diverse—encompassing insects, spiders, and crustaceans.

Narrower Taxonomic Levels

Taxonomy becomes increasingly specific as organisms are grouped by shared traits:

• Phylum: Chordata – animals with a backbone and spinal cord.
• Class: Mammalia – warm‑blooded vertebrates that nurse their young.
• Order: Primates – species with relatively large brains and complex social structures.
• Family: Hominidae – great apes, including humans, characterized by upright posture.
• Genus: Homo
• Species: Homo sapiens

For modern humans:

• Domain: Eukarya – membrane‑bound nucleus.
• Kingdom: Animalia – multicellular, heterotrophic organisms.
• Phylum: Chordata – presence of a vertebral column.
• Class: Mammalia – mammary glands for feeding offspring.
• Order: Primates – complex cognition relative to body size.
• Family: Hominidae – bipedal locomotion.
• Genus: Homo – shared genetic lineage.
• Species: Homo sapiens – modern humans.

Are Viruses Living Organisms?

Viruses occupy a gray area in the definition of life. They possess genetic material and can self‑replicate, yet they lack cells, metabolism, and independent growth. Ongoing research seeks to determine whether viruses can respond to environmental stimuli in a manner comparable to living organisms.

Organismal Ecology

Organismal ecology examines how individual organisms behave and physiologically adapt to their surroundings. It overlaps with population and community ecology, but focuses on the intimate interactions between a single organism and its environment.