A mummy was a corpse with exceptionally well preserved skin and dried flesh. This can be due to environmental or artificial means.
Vulcans practiced mummification as early as the 9th century BC, with the remains of Haadok at P'Jem being one example. (ENT: "The Andorian Incident") The mummified body of the Vulcan Kolinahr master T'Klaas was preserved in the T'Karath Sanctuary. (ENT: "Awakening")
Klingon mummies were identifiable by a particular mummification glyph. (Star Trek IV: The Voyage Home) Some entrepreneurs on Maranga IV claimed to have Molor's mummified head on display for a fee of 50 darseks per person. (TNG: "Firstborn")
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In the UK comic strip "Planet of the Dead", the toad-creatures of planet Selkye resembled mummified toads, while in the Gold Key comic "The Mummies of Heitius VII", mummies that acted similar to the Borg were found on Heitius VII.