An organism is an individual living thing capable of growth, reproduction, and responding to its environment. Bacteria, mushrooms, oak trees, elephants, and humans are all organisms. The definition sounds simple, but the edges get blurry. A colony of ants can be viewed as a single superorganism. A virus is not usually considered alive because it cannot reproduce without a host. Whether something counts as an organism depends on the criteria you apply: metabolism, growth, reproduction, response to stimuli, and cellular organization.

Organisms are made of cells. Some are single-celled, like bacteria and amoebas. Others are multicellular, with specialized tissues and organs. Every organism interacts with its environment and with other organisms. It takes in energy, releases waste, and responds to changes around it. Organisms also modify their surroundings. Earthworms aerate soil. Beavers build dams. Corals build reefs. These modifications shape the conditions for other organisms, sometimes for thousands of years.

Characteristics of organisms

• Made of one or more cells.
• Grow and develop over time.
• Reproduce and pass on genetic material.
• Respond to stimuli and maintain internal balance.

Organisms are not isolated. They exist in populations, communities, and ecosystems. No organism lives entirely alone.