What are Stem Cells?

World English Dictionary
stem cell
— n
  histology  an undifferentiated cell that gives rise to specialized cells, such as blood cells
Collins English Dictionary - Complete & Unabridged 10th Edition
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Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

(from: http://stemcells.nih.gov/info/basics/pages/basics1.aspx - last accessed March 19, 2014)