This word has two meanings, though by context, there is no way to mistake them.
Among the Mythos, a Sylph is a member of The Dream, these beautiful creatures resemble air spirits. They have control over gentle breezes (nothing storm-like), and – like all members of the Dream – feed off human dreams in sleep. (Don’t worry. That part benefits us.) Closely related to both Naiads and Dryads. If you meet one, you don’t have much to worry about. They’re barely part of this world, and communicating with one is a little like trying to be heard when whispering into the wind.