A doorstop was a small object used to keep a door in either the open or shut position.
In 1953, after expressing his disdain for the stale doughnuts found in the office of the science fiction magazine Incredible Tales of Scientific Wonder, Herbert Rossoff stated his intentions to leave Incredible Tales for its rival, Galaxy, saying he bet they knew the difference between a doughnut and a doorstop. (DS9: "Far Beyond the Stars")
In 2373, Doctor Beverly Crusher activated USS Enterprise-E's Emergency Medical Hologram and asked him to create a diversion for some Borg drones that were about to breach sickbay. The hologram protested, saying he was a doctor, not a doorstop. (Star Trek: First Contact)
Also that year, Jadzia Dax regarded the piece of technology that Worf had "ripped (it) out of the wall" (possibly a Vorta computer console) and sarcastically asked him whether they should use it for a doorstop. (DS9: "The Ship")