Technically, “limen” is the threshold for perception. So, a liminal thing is something occurring at the minimum threshold for perception. For example, a liminal sound is one that you can just barely hear – it’s right at the edge of what is perceptible and what is not.
(Something that does not cross this threshold is “subliminal.”)
More generally, the word is used to explain concepts and things that are between two states – sort of a “half on” situation. Like a “liminal friendship” or “liminal credibility.”
Why I Looked It Up
I weirdly encountered it a number of times in quick succession, but here’s an example from a review of Emily Ratatowski’s book:
My Body sits in this liminal space between reappraisal and self-defense.