What does it mean to feel lonely?

Painful I think, like a longing.
But I have a different take on it than all the typical scenarios. Maybe not for every situation -

"social anxiety or trust issues still revolve around the issue of selfishness. Because they are so focused on their own emotional stability and avoiding rejection and betrayal, they inevitably lock themselves into their own loneliness. Their self-focus leads to their own imposed isolation."

Maybe harsh, but think about it.

It’s like being consumed by a shadow. You interact and are surrounded by people and activities. You go through the motions of life. But, you feel, in your soul, unseen and unheard. You cannot quite find your place in everything happening around you. It’s a dark solitude that settles within and creates a feeling of loss of one’s validity.

I have felt loneliness a few times for short periods when separated from my family on business. I felt it every day in college when my girlfriend was miles away. I feel empty, totally empty, like part of me is missing.

I would say it occurs when you realize you would like someone to talk to, or someone to do activities with, or just someone to keep you company...then you feel the sad pang of loneliness.