I decided to do the no-sh** thing and search variations of the letters of Sylveon's name before the 'eon' portion. All the other evolutions are named in such a manner as to relate to their type. The least immediately understandable is Umbreon. Umbreon's name is connected to Umbra, which is the name for the world of darkness.
So I took people's ideas about Sylveon's type, i.e. it may be a new Light type or maybe it's a bug type or flying.
So I researched variations such as Sylv, Sylva, and Sylvan. With context additions like light, realm, or nothing.
The only thing I found was Sylvan. It refers to an "association with the woods" to quote Wikipedia directly.
You can search it yourself, obviously, but since Eevee already has a grass type it would instead relate to a bug type.
YET! An even more interesting possibility exists. It may be that a new type relating to fairy or mystical pokemon may be getting established.