1 Ghost: So how are we supposed to stop this wizard from blowing up the mountains?
2 Mordekai: Drain all the mana he's gathering.
3 Mordekai: Don't ghosts have an energy drain attack or something?
4 Ghost: If we did, I wouldn't be talking to you...

In early version of Dungeons & Dragons, ghosts had a special attack known as an energy drain, which is jargon for an effect which removes levels of experience from a character, making them weaker in all of their various skills. In more recent version this has shifted to a more specific ability drain, which merely makes them weaker in such things as strength or intelligence.

I'm not really sure where this characterisation of ghosts came from. As far as I'm aware, nobody who has (allegedly) encountered a ghost has come out of it with reduced skills and abilities.

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2024-01-27 Rerun commentary: In the current 5th Edition of Dungeons & Dragons, ghosts no longer have any ability to drain victims of any skills, abilities, or anything like this. They just do damage when they touch you. They do have a "horrifying visage", which has the possibility of causing anyone seeing them to age 10 to 40 years - though assuming this doesn't happen more than once (or, like, 100 times for elves), this is going to have basically zero game mechanical effects. They can also possess victims, taking control of their bodies, but this is nothing like the offensive abilities of older editions.