The painful sense of low spirits that comes from feeling cut off from companionship or connection.
The painful sense of low spirits that comes from feeling cut off from companionship or connection — distinct from the mere fact of being alone.
The distressing feeling of a gap between the connection one wants and the connection one has — “a person can be lonely only if she feels lonely,” even in a crowd. It is sharply distinct from solitude, the neutral-to-positive state of being alone (which can be welcome), and from isolation, the external social condition of separation (one can be isolated without feeling lonely). Lighter than desolation, which fuses loneliness with grief and ruin.
From lonely + -ness; lonely from lone (an aphetic form of alone, i.e. “all one”) + -ly.
A documented shift: loneliness is attested from the 1580s meaning the objective “condition of being solitary”; the modern painful-feeling sense (“dejection from want of companionship”) is dated to 1814 — so the emotional meaning is comparatively recent.