A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
For the purposes of a presentation, you could keep the "hidden" slides in a separate PPTX file and link to the file from your main presentation.
But how thoroughly do the slides need to be hidden? Totally invisible to anyone who receives a copy of your file(files), or just invisible as in "Out of sight until I want to see them"?