Currently the most defined word on the
Urban Dictionary, as well as
one of the most misunderstood words of all time in general. In short, it's a music genre starting with Rites of Spring, a
post hardcore band from the 80s that wrote more emotionally mature lyrics than the rest of the punk scene at the time. While Rites of Spring had no idea that they were going to start an entire movement at the time they more certainly did.
Currently there are many diverse subgenres (including one that stem from the genres of metal, rock, rap, and techno) of music labeled "emo" and all are acceptable as long as people like listening to them.