A hard-light hologram was a hologram which was solid.
Strax's fighting drone was an example of a hard-light hologram. In 1894, Strax tinkered with the drone causing it to malfunction and begin roaming Paternoster Row. Most of the residents thought that they were being hunted by a Ghost and blamed Madame Vestra for it. (PROSE: The Spectre of Paternoster Row)