Most of the cells in multicellular organism undergoes cell division through Mitosis.
While most of the single celled organism reproduces through binary fission by mitosis.
In both cases,the offspring are very genetically similar to the mother cell.
But one is defined as Cell division and other is defined as Reproduction.
Why?
Both cells from unicellular organisms (for example, E. coli) and multicellular ones (humans, for example) divide by mitosis - you are correct. The daughter cells are not just very genetically similar, they are identical (if everything goes right, which it usually does). Both of these instances are called "cell division". It's also called "reproduction" in unicellular organisms because the entire organism just reproduced, in an asexual manner - it went from one to two copies of itself. On the other hand, many multicellular organisms (and some unicellular) can or must reproduce sexually. This involves (generally) at least one partner and a cellular process known as meiosis, which produces haploid gametes, which then meet somehow, one gets fertilized by the other, and the resulting diploid cell begins mitosis again.
