What is the pinching off of the cell membrane that creates two new cells (after mitosis) called?

Cytokinesis.

Some people may not be too familiar with this stage but this is the time during cell division wherein the parental cell is divided into two daughter cells. This concurs with the two types of cell division namely mitosis and meiosis. These cell divisions happen in animal cells.

The movement that occurs during cytokinesis will make sure that the separation of chromosomes is possible. If cytokinesis does not occur during mitosis, this will surely be a problem for the cell because it will become too big and it would barely function. There would also be some cells that will contain nucleus but some will not.