A holo-scan was a scan that produced a holographic image of something real. They could be produced using a holo-imager. (VOY: "Latent Image"; DS9: "Meridian")
In 2371, a man named Tiron convinced Quark to create a holo-scan of major Kira Nerys. When confronted about this, he claimed that he was just recording an image of the Promenade to send home to his mother. (DS9: "Meridian")
In 2373, The Doctor became exited with the potential of his holo-imager might hold in the pursuit of medical knowledge. Among the holo-scans he took was one of Harry Kim. (VOY: "Latent Image")
In 2399, the Emergency Hospitality Hologram aboard La Sirena was able to rebuild Jean-Luc Picard's study at Château Picard to its exact specifications using holo-scans provided by Zhaban. (PIC: "Absolute Candor")