1)The "headbutt heard 'round the world", performed by Zidane on Materazzi in the final match of the 2006 World Cup. A combination of Zidane's name and "hadouken", a technique found in the Street Fighter video game.
2)Any powerful headbutt to the chest which results in the victim being knocked down/unconscious
3)To perform the above maneuver.
"Dude, the Zidouken is the new Chuck Norris roundhouse kick."
"They were just horsing around until Harry flattened John with that Zidouken."
"Don't make me Zidouken your ass! ZIDOUUUUUUUUUKEN!!!!"