I'll quote Clephas because I'm lazy:
"My favorite genre, as most of you probably know, is chuuni. Chuuni is a genre where the protagonist is generally at the center of events, able to effect those events, and the story is generally filled with melodramatic plot and spoken lines, meant to be taken absolutely seriously. An example of chuuni anime, for example, would be Fate/Zero or Code Geass (to give you an idea of what to look for). Chuuni comes from the word 'chuunibyou', which is a common stage of development for teenagers, when they desperately want to believe they are more than they are and sometimes even convince themselves they are more than they are. Many people who get addicted to fantasy and science fiction tend to get addicted during this stage of their lives, and it is also a stage of life that people generally look back on with embarrassment once they get over it. This is the reason why the Japanese stuck 'byou' onto the end of the word, which has the same kanji as the one for disease, lol. "