Stem cell

Definition
noun, plural: stem cells
(1) An unspecialized cell characterized by the ability to self-renew by mitosis while in an undifferentiated state, and the capacity to give rise to various differentiated cell types by cell differentiation.
(2) A cell that has not yet acquired a special function.
Supplement
Stem cells give rise to various cells when they acquire special functions through a process called cellular differentiation. They are found in fetuses, embryos, and certain adult tissues. In a developing embryo, stem cells may differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells act as a repair system replacing tissues damaged by disease or injury. They also maintain the normal turnover of regenerative organs such as blood, skin, and intestinal tissues.
Stem cells, such as embryonic stem cells, can now be grown in vitro for use in research or in medicine, although, their use remains controversial.