An organism is a living entity that can carry out life processes such as growth, reproduction, and response to stimuli. Organisms can be single-celled, like bacteria, or multicellular, like plants and animals. They are made up of cells, which are the basic units of life. Organisms interact with their environment and other organisms in various ways. They obtain energy and nutrients to survive, often through processes like photosynthesis in plants or predation in animals. Each organism plays a role in its ecosystem, contributing to the balance of life on Earth.