by definition it's a between space, like going from one place to another. in practice it's a space that should have people in it but doesn't. think an empty mall or indoor swimming pool.

the backrooms are probably the most popular example of a liminal space

In the basic definition, it's a space between spaces. A space that only exists for you to move from one space to another. Like a corridor or stairway. Somewhere you're not meant to stay.

https://lemmy.world/c/liminalspace

You watched Severance? Many of the areas in the show are liminal spaces. Always a bit creepy and odd and something just feels off when you're in them. You can't put your finger on what it is but it's not quite right.

This should help: https://www.youtube.com/watch?v=gp-2M_3HwFU

A liminal space is some sort of locale that we usually only experience in states of transience, where staying is strange. Something that represents a border or state that you simply pass through between two more permanent states. Waiting for the bus at night. Your residence just before dawn. An empty mall or office building where there are only remanent signs of human presence. The in-dev version of a video game where characters are either absent or just placeholders. gm_bigcity. All the Kane Pixels shit. A place where reality feels slightly altered, and your subconscious is ringing all of the alarm bells because existing there is just wrong.

limen was the Latin term for "threshold"

It came from a 4chan creepypasta about noclipping out of reality

My personal guess was the writer was a philosophy of mind student or psychiatry student - the most likely place a young person would encounter the term.