Simple Definition:

Binary fission is a type of reproduction in which a single organism divides into two identical offspring [copies of the parent]. This process is commonly observed in bacteria and other single-celled organisms.

For example, bacteria reproduce through binary fission. They first duplicate their genetic material, then divide into two separate cells, each containing a complete set of genetic material. These two cells are the offspring of the original bacterium.

Very Simple Definition:

Binary fission is a way for some tiny living things to make more of themselves. They split into two parts that are exactly the same. Bacteria are an example of tiny living things that do this.