The rigorous definition of a stem cell requires that it possess two properties: self renewal and unlimited potency. Self renewal means the ability to go through numerous cycles of cell division while maintaining the undifferentiated state. Unlimited potency means the capacity to differentiate into any mature cell type. In a strict sense, this makes stem cells either totipotent or pluripotent. Multipotent and unipotent are also used to define stem cell potency. These properties can be illustrated in vitro using methods such as clonogenic arrays where the progeny of cells is characterized.