A Japanese slang term which roughly translates to "Middle School 2nd Year Syndrome". People with chuunibyou either act like a know-it-all adult and look down on real ones, or believe they have special powers unlike others.
This is a common stage in growth; for most people, it happens--you guessed it--around the 2nd year of middle school (Grade 8 in Japan). However, the problem is, there are some grown-ups who have this symptom.
There are 3 main types of Chuunibyou. DQN-type pretends to be antisocial even though they aren't, or can't be and has made-up stories about antisocial behavior. Subculture-type prefers subculture or minor trends just to be different and have the "cool" factor. Evil Eye-Type admires mystical powers and pretends to have one of their own, to the extent of establishing an alias just for that.
And if you think you have one of these or want more information in the subject, a good idea is to read the "Chuunibyou User Manual" (中二病取扱説明書, Chuunibyou Toriatsukai Setsumei Sho) by Saegami HYŌYA.
If you tl;dr I'm going to clobber you.
An example of DQN Chuunibyou:
"Look at all those bandages wrapped around you! What happened?"
"I had a bunch of gang fights."
"Come on, I know you're too kind to be a gangster."
An example of Subculture Chuunibyou:
"When I collect Pokémon cards, I only collect ones that are imported from Japan."
"Can you read all those?"
"-_-; S-sure I can..."
An example of Evil Eye Chuunibyou:
"Don't take that bandana off my left wrist. It's sealing an evil spirit inside me and if you take it off I may go berserk."
"Dude, calm down. You just have chuunibyou."