A figment of the imagination usually made up by little kids who dont have many friends or have a hard childhood life. Or by some insecure teenagers who have to pretend they have friends to impress people. Also made up by insecure friendless adults who pretend like they go out with someone when its all imaginary and made up in thier head.
Sal pretended to have a friend to impress people. People ended up finding out the imaginary friend was a lie and just an act of compensation to make up for his dull life.