In short, cells that, by division, generate neuroblasts and gliablasts in nervous system. Also known as a precursor cells, they are regarded as being different from stem cells. While progenitor cells are descendants of stem cells, they differ in that they are more constrained in the ability to differentiate or to renew themselves. Moreover, stem cells can replicate indefinitely while progenitor cells do so only a limited number of times. The difference between these two types of cells is still a matter of some debate.
See Cell, Cell theory, Differentiation (embryology), Gliablasts, Neuroblasts, Proliferative cells, Stem cells