Humans, dogs, lions, fish and any other living being are all examples of organisms. By definition, an organism is any contiguous living system or being. Even plants are examples of organisms.
When many people think of organisms, they think of bacteria and fungi, which are microorganisms, or very small organisms. Other defining characteristics of being an organism include some level of response to stimuli, reproduction, growth and development, and self-regulation, which is also known as homeostasis. Unicellular organisms consist of only a single cell, while multicellular organisms can contain trillions of cells like most animals do. In complex multicellular organisms, cells are grouped into tissues and organs that each carry out certain functions.