It is the process by which cells increase in number at a very high rate.
Under ideal environmental conditions, yeast and other unicellular organisms divide as rapidly as possible, multiplying from one cell cycle to the next without taking much time.
This uncontrolled cell proliferation is also seen in the cleavage divisions of the early animal embryo, or when cultured mammalian cells are given unlimited space and nutrients in a lab controlled environment.
In multicellular organisms, the rate of cell division is strictly controlled to make sure that new cells are produced only when required.
For example, in early animal development, the pace of cell division varies majorly in different regions of the embryo, providing one mechanism by which the sizes of different organs are determined.