1) To literally be in the presence of no other person.
2) To be surrounded by people, but said people are largely inconsequential, in that you take no interest in them and vice versa, and so the situation is as good as being alone.
It can lead to intense feelings of hopelessness, and depression. One may feel that they will forever be alone. It's a feeling commonly associated with the inability to make friends or meet a girlfriend or boyfriend.
"Damnit. Since I moved into University, I feel so alone."
The act of masturbating, but instead of already being horny and just feeling like a wank, you have to touch yourself a lot just to achieve a boner. Thus, the achievement of an erection and subsequent ejaculation is wholly manufactured and processed, unlike the natural, and far more common "horny wank", customarily used to alleviate "blue balls". This processed meat is often called a "cold wank".
Dude A: I was so bored last night, I had some processed meat.