Slang; A word used in the FGC (Fighting Game Community) to describe someone who is upset. (mainly because of their loss to someone.) Though the word's primary use lies in the FGC, it can also be used in any other context to describe being upset.
Mike Ross gets really salty when he loses in friendlies.
I'm hella salty that Combofiend got 4th at Evo yesterday.
My mom made me stay home and clean my room so I couldn't go to the party. I was so salty.