A mummy is a dead body that has been preserved either intentionally or unintentionally due to exposure to chemicals, extremes cold, very low humidity, or a vacuum.
Vulcans would often bury deceased Masters underneath their monasteries and other sanctuaries. Master Haadok was buried in the catacombs underneath the P'Jem monastery. (ENT episode: "The Andorian Incident") The Vulcan Master T'Klass was buried underneath the T'Karath Sanctuary when he died in the 4th century. His identity was lost over time, until his remains were identified in the 22nd century by Jonathan Archer, who was carrying the katra of Surak. (ENT episode: "Awakening")
The Kantare Shilat was ejected from his ship into orbit over a planet just before his ship crashed, and he died in the escape pod. Several years later his mummified body was recovered by the crew of the Enterprise. (ENT episode: "Oasis")
One case of unintentional mummification took place on board the USS Pegasus in the mid 24th century. After seizing control of the Pegasus an explosion killed the crew members who remained on the ship. As a result parts of the ship were open to space for several years. The bodies of the crew members in these sections were preserved perfectly in this environment. (TNG episode: "The Pegasus")