Using the right words to describe an individual living thing is surprisingly difficult. The consensus is that an organism is a life form that can react to stimuli, grow, reproduce and maintain cellular equilibrium.

Classification systems bring order to the millions of amazingly diverse organisms on Earth. The history of biology traces back to the ancient Greeks and Aristotle’s classification system of plants and animals based on outward characteristics.

Organism: Definition and Characteristics

An organism is a single individual living thing or living being. Organisms can be simple, single-celled life forms like bacteria or complex multicellular living beings with parts that cannot survive independently.

According to Merriam-Webster's online English dictionary, an organism can be defined as an individual living being that carries on life functions via separate but interdependent organs.

European biologist Carolus Linnaeus developed a formal taxonomy in 1753 to group plants and animals. The Linnaean system of classification helps scientists communicate their findings without having to go into elaborate explanations of the particular organism being referenced. New words are continually needed to describe recently discovered species.

Domains of Organisms

Organisms are arranged and classified by traits, characteristics and DNA analysis. The broadest unit of classification is the domain. Life is divided into three domains: Bacteria, Archaea and Eukarya.

• Eukaryotes: These are organisms with a defined, membrane-covered nucleus. Protists, fungi, plants and animals, including humans, are classified as eukaryotic organisms. Even though these organisms look very different, they perform all the functions of life and share the defining characteristic of a membrane-bound nucleus, organelles and cytoskeleton.

• Archaeans: These are prokaryotic organisms, meaning they don’t have a nucleus but perform major life functions, such as digestion and reproduction. Also called extremophiles, these life forms have adapted to the harshest conditions imaginable. For instance, methanogens produce methane and can live in places like sewage treatment plants. Thermophiles live in hot springs and thermal vents.

• Bacteria: Bacteria such as cyanobacteria are prokaryotic organisms that lack a nucleus but perform life functions. In the late 1970s, American scientist Carl Woese made the stunning discovery that bacteria and archaeans are genetically distinct groups of organisms with unique genetic codes.
  

Kingdom and Phyla

Domains are further divided into kingdoms. Eubacteria and archaea used to be lumped together in a former kingdom called monera until critical distinctions were discovered. Currently, there are six generally agreed-upon kingdoms: archaebacteria, eubacteria, protists, fungi, plants and animals.

Kingdoms are divided into phyla. There are almost three dozen phyla in the animal kingdom alone. The number of phyla change as new species are added and existing species are reclassified. The largest phylum is Arthropoda, which includes millions of species of insects, spiders and crustaceans, for example.

Narrower Subdivisions

Organisms are further divided into increasingly smaller units based on similar traits or characteristics.

For example, the Chordata phylum includes the class of mammals, which can be subdivided into the order of carnivores, for instance. Orders break down into families like Felidae (cats). A family, such as the Felidae, is subdivided into genus and species, like Panthera leo (lion).

For example, here is the taxonomic classification for modern humans (Homo sapiens):

• Domain: Eukarya – membrane-bound nucleus.
  
• Kingdom: Animalia – multicellular organisms, consumes food.
  
• Phylum: Chordata – backbone with spinal cord.
  
• Class: Mammalia – nurses babies.
  
• Order: Primates – larger brains compared to other animals of the same size.
  
• Family: Homindae – upright posture.
  

Are Viruses Living Organisms?

Scientists debate whether viruses meet the definition of a living thing.

On the one hand, viruses have genetic material and perform life functions such as self-replication. On the other hand, viruses are not comprised of cells, and they don’t metabolize food, maintain homeostasis or grow larger.

Research is ongoing to determine if viruses can respond to stimuli.

Organismal Ecology: Definition

Advances in biology have led to exciting fields of specialization, such as organismal ecology. The definition of organismal ecology is the study of organisms’ behavior and physiology in response to environmental conditions.

Other related fields include population ecology and community ecology.