A cell is the smallest unit of a living organism [a living thing like a plant or an animal]. It is like a tiny room that carries out specific functions within the organism. Cells can be found in plants, animals, and even in single-celled organisms like bacteria.
For example, your body is made up of trillions of cells that work together to keep you alive. Each cell has a specific job to do, like carrying oxygen or digesting food.
Very Simple Definition:
A cell is a tiny part of a living thing that does a specific job.