(covers information from several alternate timelines)
"To have a skeleton crew, you have to have skeletons."
A skeleton crew featured a minimal number of crewmen required for starship or space station operations, sometimes fewer. Skeleton crews were often formed during emergencies.
The D5-class variant was capable of operating with a skeleton crew of just four. (ENT: "Marauders")
The final version of the Xindi weapon was designed to be operated by a skeleton crew. (ENT: "Azati Prime")
In 2259 of the alternate reality, Khan Noonien Singh informed James T. Kirk that USS Vengeance had been modified to allow for a minimal crew. He later reiterated this when Montgomery Scott asked where the crew was, adding that it could be flown by one person if need be. (Star Trek Into Darkness)
In 2268, the USS Enterprise needed a skeleton crew of twenty while using the M-5 multitronic unit. (TOS: "The Ultimate Computer")
In 2287, Starfleet Command had the USS Enterprise-A go to Nimbus III, much to Captain Montgomery Scott's dismay as he remarked, "The ship's in pieces and we've got less than a skeleton crew aboard!" (Star Trek V: The Final Frontier)
In 2368, Lieutenant Commander Data suggested a skeleton crew from the USS Enterprise-D could take the Kallisko to a starbase. Captain Jean-Luc Picard turned this suggestion down, as they did not have the time to do that since they were planning to pursue the Crystalline Entity. (TNG: "Silicon Avatar")
In 2369, a skeleton crew operated the space station Deep Space 9 after an aphasia virus infected everyone except Quark and Odo. (DS9: "Babel")
In 2370, the Enterprise-D was left with only a skeleton crew under the command of Beverly Crusher while the rest of the crew were on the surface of a planet, searching for Data. (TNG: "Descent", "Descent, Part II")
A few weeks later, a skeleton crew was left to run Deep Space 9 during a plasma disruption that forced the evacuation of the station. (DS9: "Invasive Procedures")
In 2373, when members of the USS Voyager crew were replaced with Nyrians via translocation, the ship was eventually left with only a skeleton crew, with Commander Chakotay as the last remaining officer before he, too, was removed from the ship. (VOY: "Displaced")
After the Reckoning began on Deep Space 9 in late 2374, a skeleton crew remained aboard once the majority of the station's population had left. (DS9: "The Reckoning")
In 2375, when Ensign Marla Gilmore spoke to Commander Chakotay of leaving the USS Equinox in favor of the USS Voyager, Chakotay noted that the Equinox was already operating with a skeleton crew. The vessel had, in fact, lost half of its original crew shortly after its arrival in the Delta Quadrant. (VOY: "Equinox")