A TERM used to define ANY TYPE OF MUSIC with screaming in it. Screamo can be metalcore, metal, post-hardcore, even fucking brokeNCYDE (VERY shit band) could be considered 'screamo', because it has screaming in it. screamo is not an actual genre, it's a label to define music with screaming in it.
Ignorant dude: Dude, that band, alesana, is totaly screamo.
Normal guy: dude, they're totaly post hardcore, screamo means music with screaming in it.
Ignorant dude: so what is August Burns Red, then?
Normal guy: Metalcore.
Not Ignoant anymore guy: Ohhhh, it all makes sense now...