An incubus is a scary mythological character that visits people in their sleep, a kind of nightmare demon. If you dream that an incubus is weighing you down as you snooze, you'll probably wake in a panic.
Incubus comes from the Latin incubo, "nightmare, one who lies down on (the sleeper)," which perfectly describes the legendary evil spirit that crushes sleepers, triggering terrible nightmares and making them feel like they're suffocating. In the Middle Ages, not only did people believe incubi (yes, that's the plural) were real; they passed laws against these terrifying spirits. These days, an apparent incubus is likely to be explained away as sleep paralysis, or a waking dream.