Epithelial cells were cells from the lining of the Human stomach. The cells were relatively un-differentiated and non-specialized, a fact that made them ideal for the purposes of cloning.
Doctor Phlox was studying the epithelial cells of the Xindi pilot of the crashed Xindi probe and identified him as Xindi-Reptilian and the epithelial cells as scales. (ENT: "The Xindi")
In 2365, William T. Riker and Katherine Pulaski had some of their epithelial cells stolen so that they could be cloned by the Mariposans. (TNG: "Up The Long Ladder")