Bottom space refers to the mental shift when submitting to a partner during BDSM. "Entering bottom space" is making the necessary adaptions in the bottom's mind before they can really submit to their top. This is achieved by means of ego reduction such as forced nudity, a collar, humiliation, and rituals like a slave name. The counterpart to bottom space is top space. A related phenomenon in roleplay is head space.
Bottom space is not the same as subspace.