An organism is a living thing. It can be a plant [a living thing that grows in the ground and makes its own food], an animal [a living thing that can move around, find its own food, and often has a body made of cells], or a microbe [a tiny living thing that is too small to see without a microscope].
All organisms are made up of cells [the tiny building blocks of living things]. Some organisms, like humans, are made up of trillions of cells, while others, like bacteria, are made up of only one cell.
Overall, an organism is any living thing that can grow, reproduce, and respond to its environment.
Very Simple Definition:
An organism is a living thing, like a plant or an animal. It is made up of tiny building blocks called cells.