A word typically used by people in the "Ghetto" describing a person that likes to see another fail or unhappy for another to succeed. As of late, the word is now an over-used word, used by your typical liberal/conservative young, suburban soccer mom or dad.
Becky: Like OMG I learned a new word from that rapper.... Umm... what's his name? 25 cents, I think.
Tanner: Oh that is so totally rad Becky!!! What's the new word?
Becky: Hater... I think is so friggin AWESOME. I think I'm going to use it in every sentence I use.
Tanner: Me 2 Becky!!!!!!!! Let's go frolic in the park and be haters together.
Beck: Sure hater!