The word endothelial progenitor cell is used often in the literature, yet this cell is abstractly defined. The word seems to apply to any cell that has differentiated beyond a stem cell, but has not yet become a mature endothelial cell. I would like to answer question regarding the origins of an endothelial progenitor cell, identify some consistent characteristics of an endothelial progenitor cell, and define and assign a precise stage of differentiation, level of maturation, vasculogenesis capabilities, and subphenotypes.