Loneliness

What is loneliness, really? i'm just curious. Is it being alone? no, i think not. that would be alone-ness, which is different. is it a feeling brought about by not being with anyone? is it connected to belonging to someone in a romantic way, or more aptly, not belonging? i wished i knew what it was exactly, so that when i use it next, i know exactly what it means, and i wouldn't just be going off on the mouth about something i don't understand..
answers? anyone?