An exoskeleton was a hard, supportive and/or protective structure that occurred in the external anatomy of many organisms. Most commonly found in invertebrates, such as insects and other arthropods, exoskeletons were also found on some sentient lifeforms.
Tholians possessed exoskeletons composed primarily of minerals, and were susceptible to cracking or even shattering at temperatures that were lower than their natural tolerances. (ENT: "In a Mirror, Darkly")
Rasiinians featured a bony exoskeletal plate that covered their faces.
Borg drones were fitted with artificial exoskeletons known as exo-plating. This protective layer was sometimes installed directly over the clothing and equipment that the drone had been wearing when it was assimilated. (Star Trek: First Contact) The Borg exoskeleton in Seven of Nine's arm was capable of withstanding five million gigawatts of power. (VOY: "Revulsion")