Ethereal Plane
From Syra D&D Wiki
The Ethereal Plane is a ghostly, mist-filled Transitive Plane overlapping the material plane. Not truly a separate dimension, anyone within the Ethereal Plane can see the Material Plane, but it appears fuzzy and distorted, like looking through frosted glass. The Ethereal Plane, however, is invisible to the Material Plane, and creatures in one cannot ordinarily interact with those in the other. Due to the nature of the plane, one can easily "swim" through the air as though it were water.