An altered, floaty or trance-like headspace a bottom can drop into during intense play.
First, the boring but load-bearing part: subspace is a real, measurable altered state of consciousness, not a figure of speech. In the closest thing kink has to a controlled study, Ambler and colleagues (2017) put practitioners through scenes and found that bottoming aligned with "transient hypofrontality" — roughly, the thinking, self-narrating part of the brain going quiet — while topping looked more like athletic flow. Different roles, different states. Which is why the community also talks about topspace / domspace: the Dominant's version is usually described as sharpened focus and rising energy, not the melty floatiness of the sub.
The common misconception is the endorphin story. For years everyone repeated that subspace was an endorphin flood, the body's built-in painkillers. Later research complicated that: measured beta-endorphin didn't move much, while endocannabinoids (yes, the same receptor family as cannabis) and cortisol did. So the "high" is closer to a stress-plus-reward cocktail than a pure opiate rush. Useful to know, because it reframes subspace as something your nervous system does under pressure and trust — not magic, not proof anyone "did it right."
How it actually shows up: slowed or slurred speech, a thousand-yard stare, giggling, going nonverbal, losing the thread of time. And here's the part that matters more than the neuroscience — someone deep in subspace can lose the capacity to advocate for themselves. Consent negotiated up front is doing real work here, because mid-scene the person may not be able to renegotiate. That's why aftercare exists, and why sub-drop — the flat, sometimes flu-like crash that can follow hours or days later — is treated as part of the scene, not an afterthought.