A holographic template was a set of programming parameters which could be used to design holograms. It would include physical parameters of a person or character. This template could be swapped with another for the same hologram, with other attributes not defined in the template staying the same.
Holographic templates could be created using holographic imagers that captured a person's or object's physical shape and appearance. (DS9: "Meridian")
The Doctor downloaded physical parameters of other crewmembers' holographic templates stored in the holodeck database of the USS Voyager in order to impersonate them. He did this so he could deliver its warp core to two Overlookers who were holding Captain Kathryn Janeway hostage. (VOY: "Renaissance Man")