I don't get lonely often, maybe about mid mayish of last year and before that 2018, so I can't speak from the feels when I describe what it means to me, but if I had to give it a definition probably something like the inability to connect with those around you to the point where it feels like you're kind of idolising interacting with others, like a 'want' to do it, despite it not meaning much when it actually happens? Idk man