Subspace (also sub space, flying, or floating), in the context of a BDSMscene, is a psychological state that can sometimes be entered by the person bottoming in the scene.
Subspace is a term used to describe the state that the bottom's (or submissive's) mind and body is in during a deeply involved play scene. Many types of BDSM play invoke strong physical responses. The psychological aspect of BDSM also causes many bottoms to mentally separate themselves from their environment as they process the experience. Deep subspace is often characterized as a state of deep recession and incoherence. Deep subspace may also cause a danger in newer submissives who are unfamiliar with the experience, and require the dominant to keep a careful watch to ensure their partner isn't in danger. Most submissives require aftercare while returning from subspace.
During the scene, the intense experiences of both pain and pleasure trigger a sympathetic nervous system response, which causes a release of epinephrine from the suprarenal glands, as well as a dump of endorphins and enkephalins. These natural chemicals, part of the fight or flight response, produce the same effect as a morphine-like drug, increasing the pain tolerance of the submissive as the scene becomes more intense. Since the increase of hormones and chemicals produces a sort of trance-like state, the submissive starts to feel out-of-body, detached from reality, and as the high comes down, and the parasympathetic nervous system kicks in, a deep exhaustion, as well as incoherence. Many submissives, upon reaching a height of subspace, will lose all sensation of pain, as any stimulus causes the period to prolong.