Undifferentiated embryonic or adult cells that have the potential to give rise to any type of differentiated cells. There are three basic sorts of stem cells: multipotent (give rise to only specific types of cells), pluripotent (can form most kinds of tissue, but not a whole organism), and totipotent (can form a whole organism as well as the placenta).
See Cell, Cell theory, Determination, Differentiation (embryology), Ependymal cells, Equipotentiality, Progenitor cells