Loneliness
Etymology
From "lone," itself a clipping of "alone" (originally “all one,” wholly by oneself), plus "-liness." The word is young — “loneliness” in its modern emotional sense barely predates the 1800s. Before that, being alone was described more neutrally; the ache we now call loneliness gained its name as society grew more mobile and atomized.
noun · /LOHN-lee-nis/
The painful gap between the connection you have and the connection you want.
What it names
Use it for the felt lack of closeness — which is about quality and desire, not simply being physically alone.
Example
He was surrounded by coworkers all day and still went home to a deep loneliness.
Related idea
Against solitude: solitude is being alone by choice and can feel restorative; loneliness is a felt deficit, whether or not anyone is around. The proof of the difference is that you can be lonely in a crowd and perfectly content in solitude — the variable is desire, not headcount.