For that sensory threshold, it's related to the psychological concept of liminality. This is not a common word outside of technical uses, especially compared to the form subliminal, being below the threshold of perception. It was coined in 1884 from Latin līmen, threshold. I poke at this one every so often to remind myself that it is not "limnal" but has three syllables.
Spaces like the threshold of a door are liminal, lying between defined areas without belonging to either of them.