Subspace is an altered, trance-like, state of being experienced by a submissive person in a BDSM scenario. It can be accessed through sustained pain play with floggers, paddles or a dominant's bare hands. Less commonly, subspace can also be experienced by a top (it's called "top space"); they experience this as feeling more connected and focused during a scene.
Many subs liken subspace to a spiritual experience and say that it becomes easier to reach as their experience with BDSM interactions increases. That said, there are different types of subspace depending on a person's position within the BDSM relationship, and their interactions with their dominant.
Subspace may also be called flying or floating.